Haha, if you read my post in the newbie tips sticky, I provide some tips. I play 1 vs 3 hard aggro AI on locked teams if I'm not playing LAN with friends.

Some basic things include:
1. Notice the fact that the AI takes several milleniums to colonize new planets. They sometimes take upwards to 30 minutes just to recolonize a planet that they lost in the middle of their area of control. One of the strategies I suggest here is getting the capital ship with colonize planet ability as your first one and expand like mad and completely forget about defenses. In fact, the moment you get your cap ship, don't bother waiting for a full complement of frigates. Just go go go!

2. AI likes to balance its upgrades. So if you specialize in say... missile technology or superweapons (TEC HAXORZ), you're bound to have an edge. In fact, one of the more ingenious things you can do is completely forgo any fleet and military techs until you have a fully teched up economy tree. Then unleash hell upon them. It takes some tactical skill, but pays off well. This is a good strat for TEC players with their extreme economy boosts. Note: Aggro hard AI is economically suicidal. It tries to get to max level fleet techs while completely forgoing any weapon, armour, and economic upgrades.

3. The AI will send pirates at you. Don't bother wasting your money to raise bounty on them. Use this to your advantage. It's free experience. Oh yeah, and try to remember than if you have money not spent, that means that there's money not spent. That cash could be used in an investment for your empire, instead of sitting there doing nothing. In theory, the optimum amount of resources you want is actually zero. That means you are spending perfectly. You can also use excess resources and credits to manipulate the resource black market and force the enemy AI (which usually relies heavily on it) to spend more than he needs to when he can't afford it. e.g: if you're a TEC vs Advent, artificially inflate the price of crystal through your superior economy to cripple the Advent completely; $1900 per 100 crystals is insanely high.

4. AI don't know how to defend themselves. Granted, they are AI, so it's something that's difficult to do without some level of cognition. What this means is that if you bypass or wipe out their single fleet (I've never seen an AI with more than one), don't bother fortifying your position. Just go and finish him off.

5. Lastly, don't ever be intimidated in this game. Most of the time, if you see something that's unbelievable: like the enemy having 200+ ships when you barely have 60 or him having a superweapon in 10 minutes, it must mean that he sacrificed something to get that. Figure out what that is and capitalize on it. Put on your detective hat and get those scouts flying.