As an exercise physiologist and former athlete and coach, I find this is a very common problem but here's some good news...
...it is the sleep you get two
nights before a competition that makes the biggest difference in performance. For example, if your event is on Saturday, enjoy a good night's sleep on Thursday night and know you are good to go.
Just knowing this may minimize some of the stress you feel the night before the event allowing you some peace of mind and a better night's rest.
Also, some basic tips for a good night sleep should be practiced: avoid caffeine, don't eat too large of a meal close to bedtime (but a small snack which includes a bit of protein, carb and fat - like an apple and a slice of cheese) actually helps you sleep better, make sure your bedroom is dark, dark, dark.
Valerian root is a herb that helps with relaxation and sleep. That might be helpful to take on occasion if you need an additional sleep aid two night's prior.
If you do find yourself lying there, unable to get back to sleep - keep the lights off (as light signals the body to produce "wake-up" hormones) and practice some breathing exercises to help you relax and to keep your mind from wandering.
One more tip...I used to get very nervous before an athletic competition and my coach always said "that's not nervousness; that's excitement!" This actually helped...reframing my state of mind better helped me prepare for the event.
Here's to your success!
Jess Live Well