I sweep one of two ways, the first way i try is to go away from the hips, and posting your elbow, crawling up while hipping into the person. Try this first because if it fails you don't lose position. The second way i try to sweep is to swim under their hips and spin the opposite direction with your legs. This should put them off balance and make sweeping possible. I've had more success with the second way but it is a lot riskier.