Let's pretend for a moment that you are relatively new to the SINS Multiplayer scene, and you discover through your many hours of spectacular game induced auto-stimulation, that you enjoy it enough to commit to establishing a presence amongst what turns out to be a rather 'core group' of veteran players.

Further to this; lets say you are sharp enough to pick up on the adopted conventions of certain accepted standards in things like choosing teams, showing proper restraint of verbal diarrhea, etc.  Indeed, you end up demonstrating that you know how to take direction, are willing to "pay your dues", so to speak, and have the good sense to not display tendencies of applied Trollisim when it comes to online multiple personality disorder, or any of the many associated idiotic behaviors that have become pandemic to our community.

Now; given these highly parenthetical conditions, lets tip toe just a little further down the assumptive path and say that most players who've traveled along the way before you have not only been where you are, but are more than capable of recognizing newer players periods of incubation in their progress towards acceptance, into this somewhat 'self-aware' Multiplayer community.

Under these extremely far reaching and hypothetical circumstances, it would behoove those newer players who genuinely wanted to integrate themselves into the culture, to take stock of their motives and objectives, and accept as gospel these heretofore unwritten adopted conventions of accepted standards, that govern the actions of the majority of veteran players.

In what will surely be the lost cause of patron saints, I will now attempt to put to pen, or at the very least, begin a dialogue - in list form - of Sins "Dos & Don'ts".  Future editions could incorporate a list naming Violators to the don'ts sections (if allowed).

Contribute as is apropos in spirit and theme.

Edit: New Beta Guide by GoaFan77: This is an excellent guide for beginners and intermediates alike - Supah Kudohs to GoaFan77

Edit: Multiplayer Guide by Grimm: Here is what is quite possibly the most comprehensive, and clearly written guide ever!

In-Game Behavior

Do try to be courteous and respectful to those you are likely to interact with on a regular basis.  What comes around goes around, and your reputation will largely determine the type of treatment you receive.  It may take a while for your personality to acquiesce, but it will pay dividends in the long run.  

Don't prolong a game longer than necessary. Especially to the point of insisting the opposing team "Come and get me" even though you are ostensibly the last player left from your side.  It's generally accepted OK if you want to have one last "Grand Melee", but once it's clear that you're the last of the Mohicans, don't punish others because you think there is some merit to making a valiant last stand.  Even the most formidable of forces one player can field is overcome by attrition of 2-3-4 or more players to one.  Do yourself, and everyone else a favor and bow out so that those that want, may perhaps join another game.  We all realize how hard it is not to want to make that loudmouth asshat on the opposing team suffer for whatever reason, but show respect for the others as you will face them again, and you don't want to be that guy that griefed their way through a long drawn out ordeal causing brain damage to even the most intolerant villains, but everyone else in the process.  You will pay consequences in the long run if you do, and may in fact be intentionally excluded from games once your reputation precedes you.

Do suggested by Yarlen: Also; at the end of a game - if in fact most, if not all players on a team should agree it is obvious that they will not win, it is highly encouraged for all players on the losing team to agree not only to concede, but to surrender.  Surrenders are recorded as a loss, quits are recorded as drop-outs which won't look good if we [ICO] ever expose that info.  -- I don't know how big an 'elephant in the room' this is, but it does make sense.  Currently the consensus that games [played / victories] is the only "given".  Over 50% ratio is almost the benchmark, but if ICO decided to display 'non-drop-outs' in the future, a lot of players would "have some 'splainin to do!".

Do save replays!  Replays are your portals to untold riches in experience and understanding of the game.  No other single tool will escalate your learning curve faster than watching replays, especially if you start getting into games with players of progressively higher skill levels.  Scour the forums for replays posted by others.  There are some very enlightening tips and tricks you can learn from the birds eye view that replays afford. Daily replays & Feedback threads by ReD Ruby: Daily Replays & Feedback - these are great threads to get your feet wet with some replays, and critiques of your own games.  Although they may not get new replays "Daily", they still have active posting, and regular contributions.

Don't get a corncob up your rectum because someone gives you direction.  If you are bright enough, you will recognize coherent direction from loose cannon bloviation.  You will learn over time the players you need to pay attention to.  If you are in their games, it usually means they are taking a chance on you, and if they give you direction, it's because they need you to seriously consider what they are suggesting.  If you cant discern the difference between good game mechanics and poor strategy, then you are either too young, or too immature, or quite possibly both.  Listening is the greatest tool to development in this game.

(Need more in game behavior dos and don'ts here)

Hosting, Forming Games & Team Selection

Do try to be patient while getting games formed up.  When you enter a game lobby, go to "TEAM 10" and relax, the game will fill regardless if you are anxious or not.  Also, you will see most skilled players selecting "Random" as their race prior to forming teams because they generally wait to see what composition of races comprises the opposition in relation to those of their own.

Don't - jump into a game lobby and start advocating, "How bout a multi-star FFA?!" or, "What about we each get a 'Hard' AI and start?!"  No one likes incessant chattering of noobs that start "greening" in when there aren't even enough players to fill half the slots in a particular map, and talking about, "...can we just start, I'm ready to play".  Most games will be chosen in a PUG (Pick Up Game) format similar to playground style team selection. Two Captains will be determined, (by volunteering, or general consensus based on their regarded skill level), after which any "non-captain" will whisper to another "non-captain" a number between 1-10.  Both Captains will then each declare their guess of a number from 1-10, and the "winning" Captain will have the option of choosing first or second.  Captains evaluate the field of players, and predict their picking order based on the available top choices. Captains can then either pick first, if they think one standout player will afford them a strategic skill advantage, or choose to "defer", in order to assure two (2) picks that will immediately follow the opposing Captain who is obliged to choose first.  The picking order for a 5V5 is: 12221, with the "Fat Kids" chosen last.  A 4V4 picking order is: 1211  A 3V3 is: 121

Don't contributed By Ekko_Tek: Once a PUG team selection is underway you've tacitly joined the game, entered the lottery, accepted the captains.

Don't join the game lobby and then AFK for ages unless someone can message you on Steam and wake you up.

Don't bail once teams are divided and people start greening up unless your house is on fire!

Do try to populate your games with similar skill levels.  It's no fun if you have too wide a disparity in player experience/skills because team balance is difficult in most cases. suggested from Howthe: If a game says "Skilled" in the title, the general criteria is that you obviously aren't new, but also that at the very least, you can beat a single player game with a minimum of two (2) hard AI opponents.  Some hosts will have a different interpretation of the criteria, so it's best to be upfront and ask.  You don't want a lengthy wait in a game lobby to be an unwise investment of your time.  OP: It has been suggested that some "Skilled" games allow one non-veteran player on each side as an indoctrination into the community.  This would afford introductory exposure in-game for both the newer players, and the community at large.

Don't ignore common sense with respect to hosting a game.  If you are the host, you have a responsibility to run your lobby properly.  This means promoting the selection of Captains of near-as-possible-equal skills, getting the "numbers" out, and team selection underway ASAP.  No one likes the "meta-game" of "who's going to be Captain".  Establish a consensus early, and get the game off the ground while avoiding the whack-a-mole channel rodents that pop in and out.  Close a slot if you have to preserve the integrity of the lobby, and at the very least try marshaling the noobs and smurfs from turning your lobby into a fiasco of epic proportions.

Edit:

Do have game settings as follows:

"Faster" settings with "Fleet Size" Normal, "Pirates" inactive, & "Allied Victory" on.

(Need more hosting, forming games & team selection dos and don'ts here)

Getting the Most out of Replays

Do save the replay of your games - While in-game - for viewing later.  Here is a great how-to watch recorded games from Greg30007.  Also, here is a good Q&A reply on replaying tips from Darvin.

Do - contributed By Ekko_Tek: Choose one player to focus on. Something you want to learn or watch for specifically cause you want to play that race or use that starting cap ship. [remember that watching a pro replay is like watching artists paint several Masterpieces at the same time, if you get into one of these games, the replay could be worth more to you than a hundred games on your own, treat these like gold].

(Need more getting the most out of replays do's and don'ts here)

Your Role in Eco Position (No enemy on your flanks when starting the game)

Do: contibuted by Seleuceia: communicate with your teammates as much as possible...let them know when you can feed, how many people you are feeding, and ask who needs feed...always consider the possibility that you may need to fleet up or get a titan to contribute the most to your team, especially if you have multiple eco players....

Don't: contibuted by Seleuceia: assume your teammates are communicating with you properly or are putting your feed to good use...always keep on eye on your team (especially "unkown quantities") and make sure they aren't throwing away your feed...if they are, give them advice or direct your feed elsewhere...be careful about feeding suidicide players: if they don't have factories to produce units, no amount of feed is going to do much good....occasionally even exceptionally skilled players will not ask for feed when they need it, and in those cases you may need to feed them on your own initiative...

(Need more your role in eco position do's and don'ts here)

(Need other categories here)

Possibilities:

1. (So you think you can turtle? - maximizing impact in the suicide spot)
2. (Rushing - recognizing valid opportunities early)
3. (Making the most of early Titans)
4. (Proper build orders, your part in team strategy)
5. (What to look for in a Veteran Player)
6. (How to win allies and avoid trolldom)
7. (You know your a troll when...)