The main thing is to relax. I used to get very worried if I couldn't get off to sleep or stay asleep that I would feel it the next day.

Once I worked out that this wasn't helping the problem at all I just told myself that, if sleepless nights made me tired, at some point I would be so tired that I would REALLY sleep. That worked and I've tried to stick with it ever since.

I also do a version of Clint's mantra - don't let uninvited guests, in the way of bothersome thoughts, share your sleep time and that works as well. I would usually try to replace them with pleasant thoughts.