When I play online poker of late, I take notes about the other players. Their betting patterns, the type of hands they show down. How they enter a pot, how they act after the flop, the turn and the river. Then whenever I play them again I can review my notes in order to make decisions on how to play them.
I do take into account that the last actions I may have recorded might not be how they normally play. So I will add to my observations about them for future reference. It sometimes helps to know that a certain player will go on tilt when they have lost two or three large pots in a row. You can take advantage of that type of information.
There is one player that I play that I will fold pocket Kings, Queens or Jacks to if he reraises me and I do not improve after the flop. He usually has the goods. As I mentioned, you do have to be careful as some players vary the way that they play.
Do you take notes while playing online? Are your notes helpful to you? Do you ever take notes at a live game?