There is not a great deal you can do to actually stop them coming as it is a hormonal thing. It is a combination of the amount of oil produced and the lining of the pores. If you are using HRT, then look at the progesterone in the hrt section on the left and see if you are using an androgen-based one - changing this can help.
These days, it is recommended to use gentle facewash, gentle moisturiser (preferably 'non-comedogenic') regularly and an exfoliant on your chin occasionally. Don't use any harsh spot stuff or rub your skin hard, and put a bit of antiseptic on the nasty ones.