Dreams are the mind's method of 'sorting' information it has received, and assessing issues which are being dealt with by the subconcious. Nightmares and disturbing dreams often indiciate an underlying worry or stress, so check if there are any issues that are troubling you, even if they are in the 'back' of your mind most of the time - these are the times that bad dreams can crop up. Otherwise, make sure you get proper restful sleep, and try to relax at least an hour before you go to bed.