Just relax. I'm sure you'll do fine if you're not jittery or anything. I personally haven't taken a drivers test yet, but for all other tests I HAVE taken, I forget things when I'm nervous. As stupid as this sounds, take a couple deep breaths to keep yourself from getting having a panic attack or epileptic seizure or something. Check this site out:
http://www.aquarionics.com/article/name/Driving_Test_IThe person thought for sure they'd failed, but they really passed. Hope it all goes well!