Try to review a product in a different category. I try to mix them up. If I just did a vibrator, I'd do a lingerie piece next, then maybe a BDSM piece next.
It also helps if you don't write too many reviews at once. Limit yourself to a certain number of reviews a week. When you are not writing reviews, do something else, don't think about your reviews. I play computer games and watch netflix for a change some nights