What Kiko said. Also, what Ken said.
I'd also add don't get too caught up early on with moves that require flexibility you don't have. And remember that movement and position are the other 2/3 of the equation. Learn to position yourself optimally, e.g. not flat on your back; and you'll find it requires less flexibility than you think to use a lot of the Rubber Guard. Its when you try to use flexibility you don't have to compensate for poor position that you end up injured; or at least stuffed and frustrated.
Also, avoid the temptation to go all big-guy on your partners. By that I mean, take the time to put yourself in positions that require the techniques you want to use. You only get better with reps, right?

There is a terrible temptation to use size and strength to "beat" smaller guys when you're rolling. When a little guy is giving you hell, your ego starts to tell you to just beast him, get the tap already, and put this squirrelly little bastard in his place. I've run into a rash of big guys lately who've all figured out the big guy formula for winning at all costs while rolling: hip bump sweep-to-mount, smash arm to floor, crank americana; lather, rinse, repeat. That's great against smaller guys who don't know that one yet, but your ego doesn't make your jitz better. If you want to improve your Rubber Guard, you're gonna have to find yourself on your back and maybe deal with some frustration for a while.