Sins Strategy

General Tips

Scout early and often.
Build lots and lots of scout frigates. Find out where the enemy is and what he is building.

Adapt to your enemy.
Perhaps the most important tip. Don't stick to a strategy. Keep an open mind and react to what your enemy is doing.

Know the game.
Learn what counters what so you can react quickly.

Build ships over static defences.
Fleets can defend almost any point of your empire. Static defences can't. There are instances of course where static defences are helpful for instance when you have a large fleet and need a bit of extra support for your key/home planet. I recommend hanger bays full of fighters and bombers for this.

Choke points are you friends.
You cannot be everywhere at once so position ships at natural choke points.

Build a capitol shipyard as soon as possible.
You get one free capitol ship so use it! The early you get it, the easier your first few missions are and the quicker your capitol ship levels up.

Maintain an adequate fleet.
Weather you are teching up or fighting from the start, you need to defend what you have.
Aggressive

Attack early and often.
If you are playing aggressively, don't allow the enemy to relax for a moment. You are not focusing your economy so you don't want them too. You can attack with hit and runs destroying key units and structures or go in for the kill and capture his planets.

Defend your key planets!
It may sound obvious but if the game is more than 1v1 don't leave key planets undefended while you send everything you have to attack an enemy.

Dont colonise a planet until youre ready.
Initially planets cost you money. If youre struggling with credits, dont buy a planet until you can level it up to the point when it makes a profit.

Use the black market.
If you have a shortage of one resource and an abundance of either the other or credits then buy the resource youre short of!