Try a little self-hypnosis. I'm a doula and use a similar technique with laboring mothers as well.
First, tell yourself that it's okay not to sleep - just laying in bed resting is good for you too. Let go of some of the pressure to SLEEP!
Then starting with your toes, tense the muscles there for a few seconds, then release them and imagine them getting so relaxed that they are becoming heavy and limp -- sinking into the mattress.
After a few minutes, when your toes are good and relaxed, move on to your whole foot and do the same thing - tense the muscles, release them and feel them getting very heavy and relaxed.
Slowly work your way up to the top of your head with this process. If an area starts to "un-relax" just calmly go back and repeat the exercise there.
If you do this slowly and gently you probably won't make it past your thighs, but if you get to your head and still aren't sleepy, don't stress over it as that just makes the whole problem worse (I know - easier said than done!)
Another tip is to make your bedroom a very cozy and relaxing place where you only sleep. Don't work in there, don't watch TV or play on the computer. Of course, some of us don't have a choice and have to have our desk in the bedroom because there just isn't anywhere else. But if you can arrange it so that your bedroom is your "sleep chamber", so your mind will not equate that space with working.