How to reset Service Engine Soon light

My service engine soon light has come on three times in the last three months. I have a 2002 325i. Each time I take the car to the dealer, they give me the same response. The gas cap wasn't tightened enough setting off the sensor. Each time I tell them that I cant tighten the cap any tighter, it clicks an then stops. They reset the light and I am fine for a couple of months and then here we go again the service light comes on again. They reset the light with no charge, it's just the inconvenience of taking the car in. Can anyone tell me how to reset the light? Thanks