I'm kind of with Art on this. I've got crazy long-ass legs and it can be a real bitch to hold onto a short guy's leg. For starters, gi pants and ankle sleeves will help. Anything that increases friction with your legs will up your percentages (there's a gi/no-gi argument there; we'll address that later...). Second, treat it as a stop on the way to a better position. Holding on to a slippery little Gnome is hard even with good leg squeeze so don't stay there -- get to whippin' ASAP. Last but not least, pay attention to where your hips are in relation to his. When you're stretching that lockdown it should pull his whole body down and get his hips low. If his hips are higher than yours he's got room to move -- eliminate the space and he has less room to wiggle that leg around in the first place.
Attacking with the lockdown on the opposite leg, lockdown on the outside, and traditional half guard are other options as well and don't neglect your whizzer/pimp hand options (e.g. stomps to butterfly). The whip down was something I played with a bit before moving to MN and I had some success with that as well. I forget which MTS that's in; it might be 14.