I have had a colonoscopy every year for the past fifteen years, and every other year for ten years before that. I'm a bit of an expert on them.
If you use the Miralax/Gatorade prep, it's pretty easy. Six Dulcolax tablets at noon, at 6 PM start drinking the mixed solution, you're done drinking it by 7:10 (8 oz glass every 10 minutes), spend the rest of the evening in the bathroom. My best advice is to get two different flavors of Gatorade, which makes me get less tired of it.
For the Golytely prep, it's a similar drill, but you have twice the liquid to get through, and it doesn't taste very good . It's slightly salty and has kind of an odd taste. I find it easiest to make it cool but not cold, and drinking it from the glass instead of a straw, because I can get it down quicker. Have a glass of plain water handy and rinse your mouth right after - that will get rid of the aftertaste. I sometimes feel a little nauseated toward the end, and I'll go to a glass every 20 minutes when that happens.
If you're doing a magnesium citrate prep, I feel for you. Consider changing doctors - that is the most vile stuff I've ever had to drink. The prep kit I got that time had Dulcolax suppositories to use after the liquid stool was done coming out, and that was pure, burning agony since I hadn't yet learned not to use college dorm toilet paper for preps.
Speaking of which, get several rolls of the softest toilet paper you can find and a big jar of Vaseline. After you wipe each time, coat your bottom liberally with Vaseline - the stool won't come in contact with your skin, and it'll lubricate the toilet paper to help keep it from rubbing you raw.
For me, the worst is the clear liquid diet the day before. Broth is wholly unsatisfying, and jello isn't much better. Coffee helps, and I've never been told that it's off limits - in fact it's specifically listed as ok on the diet sheet they give me before the prep. Caffeine is an appetite suppressant, so it helps me feel less hungry.
I'm very resistant to Versed, so I was awake and remember the whole procedure my first few times. It was cool for a while, seeing my bowels roll by on the screen, but after a while the novelty wore off. Now I ask the doctor for enough Versed to put me to sleep, and he's learned that it really does take enough to anesthetize a horse.
In any event, the procedure is a breeze. The prep is annoying, but no worse than that.