Heat rash occurs when your sweat ducts are blocked and perspiration builds up beneath the skin, causing a bumpy and sometimes painful inflammation. It's not always clear why sweat ducts become blocked, but certain factors can contribute to it, including bacteria, tight-fitting clothing, medications and certain fabrics [source: Mayo Clinic].
To prevent underarm rashes from developing, it's important to understand what causes them in the first place. One way to avoid heat rash is to simply not go outside in hot, humid weather. If you have to be outside, try not to overexert yourself, and wear loose-fitting clothing made of porous fabric. Also, don't use powders, creams or ointments on your skin -- they can clog pores and interfere with perspiration [source: Mayo Clinic].