Before every teamfight, answer these five questions. If the answer to any is "no," don't fight:
1. Do we have a numbers advantage? (5v4 = fight. 4v5 = don't fight. Simple.)
2. Are our ultimates up? (Fighting without ultimates is fighting at 50% strength.)
3. Is the objective worth it? (Fighting over a tier-2 tower at 20 minutes is not worth it. Fighting over Overlord is.)
4. Do we have vision? (Fighting in fog of war is a coin flip. Coin flips lose games.)
5. Can we disengage if it goes badly? (If the answer is no, the fight is all-in. Be sure you win.)
The average player asks zero of these questions. They see an enemy, they fight. Just by asking these questions before every fight, you'll make better decisions than 80% of the playerbase.