Something I've done before each of my tests is to visualize how things are going to go while at the same time, going through the motions that I'd do as nage. This has really helped me be confident in my footwork, posture, etc.
Know the techniques as well as you can. Focus during the test on what you're doing. If you are corrected during the test, don't let it bother you -- just take the correction and continue. If you really mess up, don't let that bother you either, just do your best.
Nobody ever does as well as they want to on a test. So just relax and give it your best shot.