I recently started to use this sweep. I think its great. Grabbing the leg with an underhok makes a huge difference IMO, you use it too off balance him and help the sweep.
It was pointed out already but you need to remember to bring his weight onto your leg. It slides up the back and he goes on it.
Also, you have to make sure hes not posting with his arm. Trap his arm on your hip, get a sleeve grip (with the gi) or in no gi you can reach around and grab the lat, crushing his arm into your chest.