if your an....
ad carry: stay back behind your offtanks or tanks and make sure your not getting focused, Its all about the positioning.
ap carry: stay behind and poke in and out of the fight depending on your cooldowns and try not to have a wild garen appear, it is super effective
offtank/tank: cc the most powerful enemy and try to take as much damage as you can. the secret is to hold aggro. My favorite way to do this is get them to focus me as rammus and let them bet me for about 75 percent of my life, then i run like hell. I flash away and start to roll or i pop ghost and when i get distance, roll. enemies chasing during a team fight because they are greedy and want a kill lose team fights. I dont always get away but when I die, most of the time 2 or 3 others die with me.
support: never be in the team fight/ never poke into hit. your allies should be inbetween you and the enemy. get wards if your losing or not, and oracles if you can afford it. if you keep dying, get pink wards instead of oracles and when time comes to see an invis enemy just throw the ward down infront of them