Open the window or sleeping vent a bit, or turn the air conditioning vent so that it blows on your face because the effort of keeping your eyes open will encourage sleep.
Wax ear plugs fit better in the ear than any other kind. Blocking out sudden noises may allow the background noise of the truck's engine to lull you to sleep.
A sleep mask will help block out any ambient light, such as passing headlights or neon signs. They are available at drug and department stores and online and range from simple satin eye masks which are relatively inexpensive to more sophisticated and expensive molded foam goggles.
Make sure that you are warm enough. An extra blanket can make all the difference between a good night's sleep and one spent tossing and turning. The extra weight of a heavy blanket may also make you feel more secure and help you relax.
Invest in a good pillow top foam mattress to put on top of the one that comes with the tractor and trailer. A softer, deeper mattress will soften jolts into gentle rocking.