I've found the best way to handle toxic players is threefold.
1. Play with friends.
-Find people you enjoy having on your team. Seriously. Not only will friends be more forgiving of mistakes (which everyone makes.) but friends are far less likely to be vitriolic in-game. If you have a form of voice chat such as Skype, TS3, etc you also get a notable increase in both enjoyment and gameplay.
2. Be positive! Compliment people when they do well, especially opponents! A single positive player can make a game much more enjoyable for all involved. A 45 minute match with ten positive players is one of the most enjoyable experiences one can have, even if it's a loss in the end.
3. Mute the Chat use the pings. Don't berate your teammates! Feeding most often occurs when a player gets upset or angry, as those emotions tend to negatively impact how they play. Berating teammates only makes those emotions worse in many, many people. Try and understand that losing happens to everyone (yes, even challenger-tier players) and what matters isn't WINNING, but playing WELL. If you do the latter enough, the former will come.
I try and follow these three rules in any game I play, competitively or for fun.