Pimples on the buttocks are not the same as acne. Sitting doesn't necessarily cause or aggravate them. If they annoy you, you might be able to help them by cleaning daily with an antiseptic cleanser (like Hibiclens, available over-the-counter) and applying an antibiotic like bacitracin in your nostrils every night for a month. The reason for this odd-sounding suggestion is that staph germs hang out in the nose and cause no symptoms, but bathing them in bacitracin for a while diminishes their number and their tendency to spread to other parts of the body. If this fails to help enough, your doctor might try a course of oral antibiotics, which sometimes help. Over time, this problem tends to get better on its own on many cases.
Best.
Dr. Rockoff
If you are a male and wear briefs switch over to boxers to allow your skin to breathe, most importantly try to shower every single day, if you do maybe try taking a quick shower before bed to wash away any sweat and dirt that may have accumalted during the day.
Baby powder may help as it will dry up any sweat which can be a cause of pimples.
Try baby powder with corn starch and see if that helps.