What I do is try to break down their posture as quick as possible, this means holding with your feet is less of a problem. Once broken down I try working off to a side with a arm, and am playing with a Sean Williams guard.
If they posture up while I'm trying to break them down. I get aggressive about hunting for a hip bump, if they defend, find head arm grips and break them back down again, and if they posture to the side then go straight for basic sweeps.
If you are having problems with all that and they are keeping you down and you can't find anything, change grips, and get ready to go to open guard. Being stocky, you can go for a lasso and start working sweeps from there.
ie, whatever you do, don't just stay still, always be going for something, you can't be like those long legged guys who can seem to just hold position.