Pacing yourself? I try to set out what time I would like to be at and a certain move number.
Play the opening out quickly if you know it, as you might need this time later on. A tip I remember from Nigel Short Book, seems simple really.
A bad habit I have is not focusing enough on your own game, getting distracted and looking at others chess games. This can be either boredom or that you have seen all that there is in the position? Or just enough that you know you have seen more than you opponent?
Another bad habit I have is going through the calculation of unnecessary variations, when the variation is not Forcing. A bad habit of mine is the re-calculation of a line over and over again. I believe this is down to self doubt. I would say after the fourth-fifth time of calculating the line either play the move, or look for a better candidate move. Don't keep on looking for some dream line that doesn't exist yet, as if the line is not forcing your opponent could play a response to the first move you didn't calculate then basically you have lost a lot of time for nothing.
Sometimes it might be ok to play the 90% reasonable move, IM Andrew Martin spoke about this, the move that looks right, if you have already spent time looking for the master move, don't always spend all your time looking for that one master move, as even if you find it you won't have any time left for the rest of the moves. This is if you haven't much time left, depending on the time control and chess position (opening/middle/endgame), and depending on your own nerves! If you have nerves of steel and can play your next 10-20 moves under 5 minutes then you have no problem. However if like me you start to look at the clock more than the board and then start panicking, well I find my chess and game just crumbles away.