Notes from Jason Scully's Grappler's Guide.
If you have short legs and cannot physically close legs around opponent, don't play closed guard. Play open guard instead. If you can almost close your legs, lift your hips off the ground. It will give you slightly longer reach with your legs.
Follow the hands! Follow your opponent's hands with your own hands, pushing against their knuckles with your open palm. If they're already in fist form, just push against it. If they're not in fist form, push up slightly to get them into a fist. Following their hands and pushing away their grips will deny them of grips. Sticky hands! As you're doing it, always be pulling in with your legs and driving your knees in to break his posture.
Use active hips! To prevent opponent from standing up, drive your heels down and lift your hips off the ground. You want to close the gap betwen yourself and your opponent. Combine it with driving knees in to break posture works well, especially when timed with when they attempt to stand up.
Break posture before getting a good angle. Swim, flared elbows, or arm drag combined with pulling your knees in. As soon as posture is broken, angle yourself -- don't be squared up against him. This means shrimping and getting onto one shoulder, getting your legs higher on their back, while holding them down with your arms.