I tend to get passive with middle pairs. I'll pay one or two bets to see a flop but there are few situations that will make me take it past the flop. If I don't hit the flop I will hang on to them if they are top pair or if the pot was raised and the flop comes raggy. If it is checked to me in late position and the flop is paired with low cards I will bet out to try and get rid of big aces. Other than that I dump these to any bet on the turn, they just don't improve enough, or hold up enough to make them worth playing.