I have them twice a week; they always recommended that I eat beforehand. Not necessarily immediately, but don't skip a meal or anything, and maybe have a snack if your last meal was a couple hours ago. The only time my LO took longer than normal to "pass" the test was the time I didn't eat my normal lunch and planned to have it afterward.
Other than that they are the easiest thing in the world. For mine they strap on two monitors, one for baby's heart and one for contractions, hand me a button to push every time I feel her, and then I sit there and watch TV for 20 minutes. They also take my blood pressure during it.
I actually look forward to them, they're relaxing lol.