The most popular strategy by far is to get all the races to go to war with each other.

What I discovered is that it's not too bad to have races at war with you, either.  They spin their wheels throwing ships at you, only to die.  Much better than having them tech up.   Works especially well if you didn't particularly grab a lot of starbases that game.

Along with what tetleytea said: If you are in a "cold war" (which I assume means that an AI is at 'wary' or 'hostile' with you) it is sometimes helpful to get that AI in a war with someone else. Several good things can come from this:

1. The AI's opinion of you improves a bit because its too preoccupied with its current war.
2. The AI's opinion doesn't change, but it may still be too distracted to attack you.
3. Even if the AI declares war on you, its military is stretched from having to fight multiple people, giving you an easier time defending yourself, and then lauching a tactical counterstrike.
4. The AI gets distracted just long enough for you to build up a stronger military and move your ships closer to its borders (be careful not to put them too close to their borders; that'll just anger the AI).

I'm not sure how useful you'll find these strategies - I just thought I would contribute something that helps me out of a tight situation every now and again.

The stupidest thing you'll hear on this post is as follows.........

Bribe people into peace.

There are many reasons for this........

1.-Those planets that the other guy will conquer if you don't stop him, will not belong to you. You want those planets for yourself. So as soon as you start a war with him (providing you don't have allies), take a bunch of planets, then bribe a bunch of others to go to war with him to (after you have nabbed a bunch of planets). The AI will just fall (unless you have terribly stupid allies).

2.-It's easier for you to build up socialy during times of peace. If you're far enough from everyone else, you shouldn't be worried too much. But eitherway, it's best to build up your social well before going on a killing spree.

3.-The money that goes into bribes, helps raise your diplomatic relations with that race. Wether or not they're at war with you, it always helps.

There are other reasons, but I have other things to do right now. If I come up with more, I'll let you know.

I find that the early colony rush is quite important, so I would check out the many posts that discuss this. The more planets you can grab initially the more of those fantastic colony HQs you will have, and equally important, the less the AIs will have. Be very careful though because you can melt your economy down is you expand to fast. There is a fine balance to this which you'll develop a feel for over a few games.

I second the notion of getting the spin control center sooner rather than later, and putting your 7 best warcraft into orbit around that planet. Very helpful for having the AI traat you respectfully, declare war less often, and surrender to you more easily; it also boosts your overall score.

When possible I keep the AIs at war with each other; in fact I keep every AI at war with as many other AI's as possible.  There are lots of reasons for this. It retards their progression, makes it less likely you will be attacked, makes it easier to tak eover their weakened empires, but my favorite reason is that the resource mine syou missed getting in the early rush, you can get fery easily if you park a constructor by the AI resource mine and just wait for one of his enemies to come by and blow it up. You immediatly park your constructor on it, and since you are not at war, you keep it. I wind up with all the resource mines over time in this fashion.

I also make it a point to make sure I get Aphrodisiac as quickly as possible. Growing your population quickly is important to being able to out influence the AI. I then make sure I get the Tirquan module before the AI. Since the end game will revolve around getting invaded and invading others, soldiering is very important; The Tirquan training is priceless. In fact, I generally do not ever garrison my planets with any defensive ships, and I play at the suicidal setting. Why not? I like it when the AI invades me! My defensive strategy involves having a population of 14 Billion plus, and very high soldiering ability, coupled with aphrodisiacs and fertility plants. They can invade, but I outnumber and outclass them, and I can replace my losses faster than they can throw ships at me. My planets can chew up invasion fleets in the very rare occasions they sneak past my offensive fleets. This, BTW, is just my play style, but it works for me.

Mkae sure you specialize some planets into economy, industry, and research centers, but not all planets have to be specialized. Some just don't work out that way. Have a good naming system from your planets that let's you immiedately recognize your strategy for the planet later on when you are dealing with dozens or hundreds of planets (PM me if you are interested and I will send you a helpful and effective naming protocal.) Have fun!

One cleverly evil tactic for upping your war rating early on:

A cargo hull has the same amount of space as a large hull, for half the cost, and is available from turn 1.  I had one game in DA where I built a 'fleet' of paper tiger cargo hulls packed with weaponry, and successfully intimidated the AI into leaving me alone.  Combined with the Spin Control Center, they provided a very large, relatively inexpensive boost to my military score.

Buying cash is always nice, but if you are truly insane you can buy almost anything with endless repititions of that theme. Buy all their cash, then use that cash to buy ships, tech, planets, treaties, etc. After each purchase, you will need to stop and rebuy all their cash, so it's a slow, carpal-tunnel-inducing process - but it works. If you start it early enough, you can keep even suicidal AIs in check, to the point where they never expand enough to be a threat.

My current game, I bought out two empires on the first day I met them, to the point where they only had their three most populated palnets left. Given the layout of the planets, one flipped in short order, the other 5 could at any time (trying to avoid that, they both have tech I want them to research first). The only thing you can't buy this way is mining bases and an empire's last three planets.

The important thing is to catch the planets right after colonization (or invasion, if you let them get that far). The value the AI puts on a planet is based almost entirely on population; if you can get the Korath to start sporing everyone in sight, you can buy those planets for a few hundred IP, or sometimes under 100 bc.

NOTE: attempting the preceding strategy is a MASSIVE financial strain. Even the best economy will be sucked dry by this for the first two years; your only hope is to buy bc off the AIs and minors faster than you can pour them down the sinkhole that is your colony upkeep (I was losing almost 3000 bc a turn at the beginning of the second year). OTOH, last time I tried this I ended the colony rush owning all but about 30 planets.