From open guard just have both your feet on the opps. hips, control the arms at the elbow. Just constantly be repositioning, pushing his hips with your feet (this'll break his base/balance and help you set up subs/sweeps) basically just play it until you catch something..
Say I'm in open guard (controlling his arms at the elbow and my feel posted on each hip) If I'm going for the left arm I grip his LEFT arm with my LEFT arm at the tricep muscle, my right arm goes across his neck and holds his right trap muscle. My right foot then pushes the hip to break his base as my left leg climbs as high up on his back as possible while keeping him down with pressure (you act as if you want your left foot to touch the ground, just keep driving it down, therefore keeping him from posting up and slamming you) then pivot to your left use the grip your right hand has on his trap to pull his head down (keep the same grip.. This also helps keep the guy from posting up) at that point bring your right leg over the head.. then finish the submission.