You don't ask easy ones, do you Amy? I've had the same question myself, more than once. Ass can be too blatant, butt too crude, bum is ok if you're in England, fanny is ok anywhere except England, and derriere sounds way too formal or high class. Backside and rear end sound kind of country. I guess the best advice I can offer is that sometimes the best way to name that particular piece of anatomy is to not name it at all, or do so indirectly. Cheeks is a good way, if a hand is sliding down a back toward them. Globes works also, or globes of flesh, like having someone bent over and a whip/hand/etc. descending toward the firm globes thrust out by that position. Or I guess you could go completely country and refer to where the seat of the pants would be if the person were wearing any. Basically, what word to use is a feel thing, and, from what I've read of your work, your feel is as good as any I've seen. Use what feels right to you. No matter what word you use, there'll be some who agree and some who disagree. What matters most is that you are satisfied that you've used the proper words to describe what you intended. And if, by chance, you do discover the perfect way to decide which word to use, please let me know. Which word to use for the other intimate areas (breasts, crotch, etc.) would also be appreciated. Do what you do, Amy. You do it well.