Before I go on, I need to state that I weigh 145lbs so this may be a big factor in my question.

I find that when I have to pull guard, I am easily lifted from the ground if I can't lock their legs in. My question is, What can I do to keep them down with me so I don't get OMGWTFBBQ dropped on my spine? I've tried to "sweep" their legs with mine, but I just can't generate the power needed to keep them down. Any suggestions would be nice.

My second question. How does one generate the same leverage doing an armbar while facing the ground. I mean, I generate the leverage with an armbar pushing up with one side of my hips when I'm on my back. But say I get the arm while on my stomach, how do I generate the same leverage? I hope this is clear enough. Thanks in advanced.