Learn to wind down
If you're having problems sleeping when you're supposed to then the trick is to train yourself to relax.
This might not seem so relaxing, but learning to wind down before you go to sleep will help your body get to the place where it wants to shut down.
Try making a routine before you go to sleep. This can obviously be a bit trickier to orchestrate if you've already got other children but little things can go a long way so it's worth a shot.
Try reading, relaxation CDs or tapes and long baths before you get into bed. As well as this, try not to get too worked up if you're not sleeping well, it won't harm your baby but it will help to just rest as much as possible.