Danaher says:

Maximize your connection and minimize your opponents postural integrity: The biggest initial problem you will face when working from open guard is having no solid connection to your opponent. This will allow him to immediately get past your legs and pass before you can do anything effective. The second problem is that if his stance is well set, it will be very difficult to attack him successfully with either sweeps or submissions. So start by getting, whenever possible, BOTH FEET AND BOTH HANDS SET ON YOUR OPPONENT to form a solid connection that won’t allow an opponent to simply blow past your guard before you can do anything. Then make a pledge to yourself that prior to any major attack from underneath, you will BEGIN BY DISTURBING YOUR OPPONENTS STANCE/BALANCE. Do this two things consistently and you will soon find yourself a better offensive and defensive guard player.