I have been getting pimples(blackheads) or boils on my buttocks which sometimes become accesses. what can i do to prevent them? have tried antibiotics

Several things: Do your daily hygiene with NORMAL soap/water - NOT antibactierial soap. This will only kill "nice" germs - not the aggressive staph infections. Laundry underwear sensitive skin products. Apply good eczema cream over buttocks after each shower. Consider using acne body wash over affected area daily. Drink plenty of water, take daily vitamin, eat healthy. http://www.cdc.gov/mrsa/

Protect the skin: Keep the area cool and dry, loose fitting cotton underwear, and wash with mild soap. Do not traumatize the skin (for example, shaving, abrasion). Talk to your doctor. This should be investigated further.