What to do when flattened out in half guard

arlmwl says: I'm terrible at getting on my side and getting an underhook in bottom 1/2. Usually someone passes my guard, I get bottom 1/2, but then they flatten me out before I can get to my side. What should I focus on; being faster during the transition to get to my side, or should I try to slow them down with framing, knee shield, etc and then get to my side? I just feel like I'm missing some key in getting to a better position in bottom 1/2.