Play something that can splitpush. Splitpushing and slow pushing are the strongest strategies in this game. You want to play someone with high 1v1 potential, and high mobility such as a jump or leap to escape when they send someone to get you. Each role has a key element that when at its best can decide the game, but I feel splitpushing is one of the best ways (or the best way) to tip a game more into winning than losing. You wanna choose people like these, usually top and JG, but some can fulfill other roles as a splitpusher 
High 1v1 potential, easy outs, leaps, and defenses. Inhibitors are huge in this game, so splitting hard enough to take an inhibitor will draw enemy attention to that lane, leaving more opportunities for your team to over power their team at Drag or Baron. After taking an inhibitor, Group as 5, and try to 4v5 them, or if they use all 5, the lane with a missing inhibitor will do heavy damage to their base turrets.
Splitpushing is a tricky game. You wanna split when there's a 4v4 (go where the other guy isn't and push) or 4v5 standoff mid, preferably a situation where your team is sieging their tower. When you split, you always want to ward. Ward specific corridors that you would consider walking past if you were trying to get you. 1 ward is good, multiple is great. Take Scuttle when splitting. Continue pushing until you see only 3 enemy champions on the map. If you see 3 or less enemies, stop pushing and watch your wards for enemy champions. Enemy team is likely to do 1 of 3 things.
- They will send 1 person to stop you, in which you continue laning with them, or straight out 1v1 since you are a 1v1 champ.
- They will send 3 or more people to stop you in hopes they will use MS buffs and catch you out and kill you.
- They will team up as 5 and run it down mid 4v5.
You want to hope they use option 3 as much as possible, leaving a lane wide open to be taken. Make sure you are always running Demolish from the Defense Tree in your Runes. Every time you see 3 or less champs on your mini-map, watch wards, and retreat.
Slow pushing is a strong strategy in splitting. When you slow push, you want to make your wave have a higher number of minions than their wave. Usually killing all casters or all melee minions will do this, and the wave will build up and crash into the enemy tower over time. When you start a slowpush, you want to rejoin your team mid for 5v5 teamfighting, and when you win the fight, you can push the lane you slow pushed, or if you lose the fight, they will have to do something about the wave slowpushing, leaving your team more likely to stop enemy threats.
