How can I prevent acne from forming?

I don’t actually have much trouble with acne, just a few small pimples every now and then. I use Stridex, but it doesn’t seem to get rid of the acne. I just want my face to be totally acne-free. I usually get pimples on my forehead and around my nose. I don’t need any products to get rid of acne, just to prevent it from forming. Any ideas?