Bush tactics work. If your team complies, just sit in random bushes in their jungle and wait for someone to show up and get a pick. Then choose a different bush and do it again. I did this against high diamond team in a tournament i was in and it won us the game.
Punish bad positioning. Don't ever have bad positioning. Bad positioning once will wreck u. So if you want to crush your enemy, never be in a bad position.
Know your role and responsibilities of that role.
Know your priorities -- (Last) Tower > Dragon > Baron > Inhib (First)
Choose champions that fit with team composition.
Build correctly, don't follow online guides they aren't specific to your current game.
Push advantages, don't recall after successful team fight, go take something. But don't OVERSTAY. Overstaying loses games.
• Keep track of enemy cooldowns, ESPECIALLY in teamfights.. If no one is gonna peel for you, know when you can take out a high priority target before he has spells to take you out.