Ai ganging up

There are a few ways..

1) Play on an easier difficulty, easy/medium shouldn't gang up much.

2) Play with locked teams so the AIs can't form treaties (but neither can you).

3) Form cease fires early so the AIs friendly to you go bug other AIs.

4) Don't be the #1 empire, that's why they gang up!

I always make a point of completing two of each AI's missions (if there are four or less AI's), so i can get ceasy-fire and trade treaties, that gives me time to build up my empire and fleet, and decide who do i want to annihilate first. Also, if i mix AI's of different difficulties, i set harder ai's on warm colors (red and orange), so i know who are the biggest threats.

Annatar's points are great, I have done them all.    I also have done tojka's tip by making hard ai's shades of red.  Here's some specific tips I can add.

Defense is crucial in the early game for me.  An attack by a few AI factions in the game is nearly certain.  Watch for a scout, then a few light frigates, then many more light frigates with possibly a capital ship.  When you see a couple of light figates, know that a larger attack is likely coming.  Invest in hangers and repair stations at these points.  You need something there to out right kill those light frigates, and tie up the larger invasions and pirates.

Keep your number of colonies low enough that one fleet can jump quickly to them ( I like to have a fleet one jump away).  Generally this can mean having 3 or 4 colonies until your economy is up and you are building a second fleet.  Until then you will be fighting in areas that are very favorable to you due to the repair stations healing your capital ships and the present hangers and guns.  Phase inhibitors are useful too, as the enemy sustains high losses as it retreats and this can buy you a lot of time.  I also steer away from light cruisers, and long range missle boats and go right for light carriers and capitals if I can.  This is because they are far harder to kill and the repair stations are so good at repairing them.

Finally Vasari has a technology that allows a fleet to travel to any world they control from any other world they control via structures you can build (phase stabilizers).  This allows one fleet to cover your entire empire. 

Well you either have to be really diplomatic so as to make allies, or be the best and that way it doesn't matter if they gang up on you, your already to powerful.

Something fun to do is ally all of the AIs and the send in colony ships to the planets that they are seiging of eachothers and capture them. Benefit from their conflict. THis is especially good for the Advent because of their powerful culture, that way if any ally decides not to be an ally their worlds revolt and you get them.

MWAHAHAHAHAHAHA!

When I started playing this game, I had the same problem.  I found that the AI doesn't seem to attack a planet if you have enough units on that planet... so if you have about the same or greater amount of "bars" compared to the AI's, they don't seem to attack.  They also will mass near your planet before they attack and will scout you (either with scouts or frigates) before they do so.

So, just keep on eye on your border colonies.  Remember that the AI is also attacking each other.  At higher skill levels, you will have to keep multiple fronts.  And, as the other players suggested, you can also perform the missions to get them to like you.  Once you get to around 50% (about 2 missions completed, if none failed), you can usually get a cease fire.

If you aren't with the AI, then you're against them. Pay the tributes. Think of it this way- would you rather pay an opponent 500 credits to leave you alone, or would you rather face his 5000 credit fleet? The amount of trouble it spares you more than makes up for it.

Playing aggressively will also win you AI allies. I had won 3 allies simply from doing some early raiding on my neighbors. An ally is not only leaving you alone, he's covering your borders and countering other player's fleets! So keep them around, and when they clash with an opponent you can crush them both.