Taking notes on players. Here's how I do it.

I play on ACR and will put in a note on a player occasionally. Yesterday I was playing a PKO tourney and I noticed that I had put in a note on a player at my table. The note said simply, "Calling station." I got into a hand with him (I had him covered, but it was early in the tourney). I had two pair (aces and sixes) on an A-Q-9-6-3 rainbow board. He check-called the flop and turn, so when it got to the river, I remembered my note and shoved. Sure enough he called with a pair of queens and I got his bounty.

sometimes I put some notes on opponents, but in general I try to navigate the situation in the hand and based on some statistics on the opponent