Here are some of the things I've learned about this format:
1. Playing chaos in Apocalypse is not the exercise in sportsmaship that it so often is in a standard game of 40k. If you take the right formations and use your assets correctly, you can cover over two of the major weaknesses of the codex: second rate shooting and third rate mobility. Formations like the Cohort of Blood, which completely nullifies a gun line the turn it drops, or the Lords of the Black Crusade, which brings a bucket of strategic assets, are very strong.
2. As a chaos player you are, in Apocalypse, the king of the orbital bombardment. Abaddon drops an apocalyptic blast every turn; indiscriminate bombardment drops two more; scheduled bombardment gives you another three. The Chaos Conclave lets you bring another. If you place these blasts correctly and the dice don't screw you, you can cripple alot of enemies on the first turn. Three very useful combinations of assets are Blind Barrage/Strategic Redeployment, Thousand Sons/Fateweaver/Altar of Chaos, and Vital Objective/Corrupt and Despoil. The first lets you put your entire army within striking distance at the end of the 1st turn, or else forces your opponent to deploy in odd ways in order to counter it; the second gives you as many 3+ invulnerable saves with rerolls as you need, backed up by a guy with 3 wounds and a 2++ rerollable save. The only way to really get rid of Fateweaver when he's on top of the altar is to hit him with a vortex missile (you'll never get close enough to throw the grande). The third lets you double the value of an objective which, if it is protected by, say Fateweaver, the Lords of the Black Crusade, and half a company of thousand sons, your opponent will never get you off of, while at the same time taking away an enemy objective. This gives you control of 40% of the available objectives and you haven't even done anything yet; hold one more and you win. The other two can be ignored.Other assets that are your friend are Flank March (often banned because it's absurdly good for assaulting armies), Hammer Blow, and of course the mighty Vortex Grenade. Don't take major possession. No matter how cool it might be to possess an enemy character, the fact that they choose which one completely nullifies this asset. Letting your opponent place your models is never a good idea, and they'll probably just throw away some 100 point chump anyway. It's not worth it.
3. There is no problem that can't be solved by enough chaos terminators. They are your go-to unit as a chaos general. The more the better.
4. The tyranid bio titan is crazy good at killing other titans. If you're playing bugs, take as many as you own or can borrow. If you're playing against tyranids, don't bother bringing your own titans; hierophants smoke them without even thinking about it. As a chaos player you have atleast one very strong counter to the hierophant; if you own enough terminators you can give them all combi-plasmaguns, drop next to this guy, and shoot him full of holes in one turn. Pretty strong.
5. Lascannons are in Apocalypse what meltaguns and missile launchers are in standard 40k. If you're playing chaos or an imperial army, bring as many as you can get ahold of.
6. Take as many destroyer weapons as you can. They're amazing
7. Plan plan plan. Nothing is worse than realizing you can't win a game of apocalypse on turn 3, and having to play for another 5 hours. Know what formations and assets are available to your opponent, so you don't have to learn about this stuff while he's beating you over the head with it.
8. Always take a disruptor beacon or something that gives you an equivalent effect, and always plan on your opponent doing so. The Chaos Conclave is my preferred method of doing this, because the beacons are only really important on turns 1-3, and it's very hard for most opponents to get rid of 8 chaos lords that quick. The reason you always want a disruptor beacon is that it completely hoses opponents who use strategic reserves without planning for the beacon. An enemy unit that's placed 6 feet away from the nearest objective is essentially out of the game, and all you have to do is roll a 4+ to do it. There aren't many places in 40k where you can get those kinds of odds. By the same token, if you're using strategic reserves, you have to plan on your opponent having this thing, if only to protect yourself. If they take it in its asset incarnation your best bet is to knock it out with an orbital bombardment, as its pretty fragile and likely to be well-guarded and/or hidden. But you can't hide from an orbital barrage.
9. Never, never, never take the doombringer annihilation force. If you want to drop a crazy number of terminators, just do that. You don't need the formation. The fact that its keyed to a particular objective, and your opponent knows which one because you have to keep hitting it with an orbital barrage, lets your opponent plan for it's arrival. All they have to do to kill the whole formation is spread out, leaving the terminators with no room to deploy, and hence forcing them to roll on the mishap table, where the unit is likely to be wiped out. It's difficult to recover after losing an entire formation to something dumb like this.
10. Speaking of strategic reserves, keep in mind the size of the board and the point values - you might be surprised how quickly a board can fill up - literally fill completely up - leaving you with no place to deploy. Not cool.
11. Plan on 1 hour for every 2000 points on the board, roughly. If you have 6 hours, don't play a 10,000 point-per-side game; you'll never finish.
12. Power armor doesn't mean shit in Apoc. Too many destroyer weapons etc. If you're not in cover, you're in trouble.
13. If you have better mobility than your opponent, deploy your objectives far away from each other. That way if he tries to go for them individually you can use your mobility concentrate your whole force on one part of his army at a time and wipe it out piece by piece. If you have worse mobility than your opponent, deploy your objectives close together so that you can keep your guys close enough to support each other and don't have to run around trying to protect them. This holds for normal games of 40k too. You should know, as you are placing objectives, which ones you intend to hold, which ones you intend to ignore, and which ones you intend to corrupt and despoil. Those decisions ought to inform your placement of the objectives. If you plan on holding an objective, for instance, it's probably a good idea to put it in cover, etc.
14. The entire format is pretty ridiculous if you play competitively and allow proxies. If you do there are all kinds of tricks possible that are evil, funny, and not at all funny, all at the same time. You can probably think of 5 in as many minutes. The only ways to get around this, that I've found, is to have someone else write both armies, or else to disallow proxies entirely. Most people don't have enough models to abuse the freedom that Apoc gives you; they would have to proxy to do the really dumb stuff.
Well, that's all for now. Cheers!