Note:
I am posting these dirty tricks as many people have asked for them, but I do not recommend using tactics that are cheesy and underhanded if you want to keep your opponents and not upset your friends. Cheesy tactics are a good way to make enemies of your opponents in real life.
The Ultimate Chaos Terminator Devastator Squad
Take a squad of three Chaos Terminators, make them Noise Marines, give two of them Reaper Autocannons and give the third Terminator a Blastmaster (this is so cheesy it should be illegal!). And the whole squad is immune to psychology and all Break tests and has a Psychic Cacophony!!!!!! Extra Sharp with some Colby Jack on the side.......
Noise Marines with Jump Pack (Mobile Psychic Headache)
I know they're expensive, but they can really mess over your opponent's psykers! Just jump them in close to your opponent's psyker and they've done their job.
Versus Tyranids (Pestacide Troops)
The very best Chaos troops when fighting Tyranids are Noise Marines armed with Sonic Blasters. They are immune to psychology and Break tests, and they have the Psychic Cacophony which does a number on the Hive Mind. Another good choice is Tzeentch Marines, Thousand Sons Marines.
Berzerker Terminators (Frenzied Adamantite)
These guys are great for taking out enemy characters and tough troops.
Three-Man Squads (more than three's a crowd)
A slightly cheesy way to use your Chaos Marines is three-man squads.
Marine Squads: 3 Man Devastator Squads (firing lines!)
You can arm them with lascannons for anti-tank, or all missile launchers with plasmas for anti-Bugs (more pestacide), or a combination of autocannons and missile-launchers for anti-personnel/light vehicles (this is especially good versus Orks and Eldar players).
Three-Man Veteran Squads
Use these infiltrated to disrupt your enemy's battle line and to channel him into killing zones. You can also use them to kill artillery crews and characters. Another use is to give the squad heavy weapons to ambush tanks and important characters.
Mekboy + SAG + Snots + Bloodthirster = Instant Daemon!
Take a Mekboy as an ally with a few Snotling bases, have him fire them at an enemy target, and have your Bloodthirster possess one of the Snotling bases in the Warp, you can have lesser Daemons like Bloodletters possess the others! This is so cheesy it stinks..... Limburger, limburger........
Editor's Note
On many sites you will see people recommending you to take allies just for the purpose of slaughtering them yourself and getting Summoning Points. This is an illegitimate tactic. You will also find references to 2+ on 2D6 Terminator armour, this is incorrect, Terminators with the Mark of Khorne only have a save of 3+ on 2D6 (this is the official ruling by Games Workshop). You will find many cheesy and sometimes completely illegal "tactics" on many of the Chaos sites but these are no replacement for good, sound tactics.