General Tip: Play In Moderation

This might sound cliche but it's what has kept me sane since I started playing again after being away a couple of years. Try to only play in batches games at a time and when you're feeling particularly competitive. Some added points only from my personal experience:

• Playing too much makes you sloppy. Just like your first game of the day is typically weaker since you need to warm up, you can start to lose focus over time and miss simple things you might otherwise notice.
• Physically, getting up and taking a break will make you play better. Even if it's only a 10 minute walk you'll come back better than before. This is particularly true for "older" gamers once you get past your mid-20s. We're not quite old enough to need naps yet, but sitting still too long can make us cranky :P
• Team Morale. I think most folks shrug their shoulders when teammates get outplayed, but it's errors in judgement that tend manifest themselves before drops in mechanical skill- and those are what cause people to flame. Overextending when the enemy team is MIA, unintentionally ignoring pings, etc. If less folks play when their head isn't in the game it will be good for those who are.
• On the flip side: taking breaks causes you to get less angry at the game when you do poorly. Don't queue up when you're on tilt! You'll tend to lose more often than not and over time will come to resent the game.
• Don't queue up just because you have free time. I mentioned it before, but League is a game best played when you're actually feeling competitive. Find some other games to play if you're bored or want to goof off with friends. Before you sit down to game try to think for a moment where your head really is and choose the appropriate game for your mood.