When you see a good move, look for a better one. Emanuel Lasker.
A fairly well known but pretty important quote. What chess players don't get, is that it also applies to your opponent's moves. When calculating a combination, or any variation, always look for powerful moves from your opponent. When it's done, look for better ones. This could save you many games. Bobby Fischer once said: My opponents make good moves too. Sometimes, I don't take these things into consideration.
This one too is important to keep in mind. Taking into consideration your opponents make good moves, and looking for them when calculating is the key to avoid blunders.