alway try to bring on players towards the end. I try to beef up my midfield towards the end if im hanging on to a lead. Always look at the motivation of the player. If a player is nervous and its around the 80th min of the game sub him. If its happening all the time try to play 5 in midfield and imploy time wasting tactics. It also helps (well it does for me) when you become strict on people making mistakes on a regular basis and when they do well try and single them and praise them. Hope that helps.