soccerdude,
Try eating your cactus with a small glass of water and a single piece of bread, preferably toasted. Whatever you do, don't consume tea or coffee with it! I'm not talking about some hoity-toity herbal tea, but rather the classic caffeinated teas... these will lower your threshold for nausea. Not good, unless you want to purge. You might also want to stay away from milk, as this can make your body produce more mucous, which you'll have enough of because of the cactus. At least I think it's mucous... it's very much like mucous, anyway. Same for orange juice, but then again, the citric acid can help with the digestion, so it's your call.
Some people say taking a few bong rips during the come up (on any drug) helps eliminate nausea, but doing this has never worked for me; it has only made things worse. Besides, when I'm having a strong trip I like to experience it to its fullest without pot.
Also, after consuming the cactus, for about 45 minutes to 1 ½ hours you should try to remain still. Sit somewhere and watch the walls move. You may start to feel sleepy. If you're anything like me, the nausea will pass quickly, the sleepiness will clear, and soon you will feel energized and expansive. Good luck!