Dirty tricks:1. When you plant a c4, or another traitor does, use voicechat to mask the sounds of the c4 beep. Good innocents will pick up on this, if you're too obvious, so be clever. Don't just sing into your mic. Start up an argument with someone, or get your fellow traitor to improv with you. Give other people chances to talk too, if you want. Just be careful not to leave silence :3
2. Shoot people with silenced pistol / flaregun from far away, when they're in groups. A lot of the time, they'll freak out and start killing eachother. You can take out whole groups this way.
3. This one's pretty hard to pull off. For example, if you see an innocent kill them, but you don't have chance to help, if nobody else is around, kill the innocent before he can ID the body. Then, type into chat (Make sure you get the spelling bang on right) "I'm with <playername>" then a second later "<playername> is the traitor!" a few times. Then ID your fellow traitors body, and be like "Haha, owned dumbass". Then everyone will think you're innocent, since it'll look just like you killed the traitor.

Feel free to justify it by saying something like he killed the other guy, and ID him too, although you'll have to try and explain how the innocent died later, if anyone checks.
4. Propkill! This is a pretty dirty one, but you can generally propkill, and get away with it. Prop flipping is especially effective. Hold out a long, flat prop, against the floor, when someone jumps on it, quickly jerk your mouse upwards. The person on it will go flying, and fall to their death. Justify this with "Well, they jumped on it", or whatever you like. This is rdm as an innocent though, so don't do it! :V Also bear in mind, SOME people will kill for this, but you can generally get away with it. The large wooden pallets, and the bikes, are especially good for the flipping.
5. Lie! If someone damamges you a lot, but you kill them, id the body, and make a big rant about stupid RDMers. Then, nicely ask the detective for a health machine. This works for me so often, it's criminal.
6. Traps! Some maps, e.g. lost temple, richland, come with traps. This is great and all, but people expect these. Set up your own! One of my favourites, is hiding some barrels the other side of a door to a room someone is in. Pop in the room, shoot them once, leg it out. They'll chase you out. Shoot the barrel and BOOM! They're dead :3 Disguiser helps out with this one a lot, so they can't get a nameshout in if they see you.