Make sure that you are raising your hips up constantly, finding that out was huge for me.
This is a very key point, the way I think of it is I try to make the opponent carry all my weight.
So get your hips as high as possible high, clamp tightly (this is where the my free leg/foot needs to be on his trapped leg, to answer a previous question).
Once clamped, then hang all your weight off that clamp. I think of clamping tightly, then sagging the rest of my body like an pet that doesn't want to be picked up. It seems if you're too rigid, it will be a bit easier to counter, so clamp and hang like dead weight, staying loose until time to sweep.
Many will just fall over at that point. Then think of using your clamp and weight to open his knee up widely. If you also have the free leg at the ankle, he is almost sure to fall.
If not, the "triple pump" will finish them off.
As any control position, it's a good idea to use beginners to practice your control. Don't go for the sweep right away, just see if you can hold it for 30-60 seconds and let them spazz. Pay particular attention to how they react and try to feel when they are off balance. You can gradually build up to higher belts and repeat the exercise ("in-roll drilling"). Doing this roll after roll will make you feel super comfortable in the position and allow you to feel when the moment for the sweep is there.