I enjoy playing guard, and I am sometimes flattered when people ask me for advice. By explaining different aspects of how I approach the guard, I've come to realize there are three aspects which really do matter, regardless of what kind of guard you choose to play. There is, of course, generic advice such as moving your hips to create angles, get your favorite grips, etc, but I believe there's something more to it.
So let's stop beating around the bush, and get right to it. I call it the three P's : Push, Pull and Posture. A good guard will allow you do to exactly this : Push your opponent, pull him and control his posture. These three aspects are crucial to get your guard to be successful. Let's examine these in depth.
Pushing is mostly a defensive maneuver, but not only. You absolutely have to be able to push your opponent away when you get in bad spots, and your angles are all wrong, or your grips are funny. This allows, from a defensive point of view, to stuff different passes your opponent might try. On the flip-side pushing is awesome because it creates a reaction you can capitalize on. Ok, I will let you in to a little secret of the purple belt and advanced blue crowd. Don't tell anyone, but we love momentum, especially when we get it for free. Why ? Because we can use it to our advantage with proper timing and technique. If we fight similarly skilled opponents, it gets much harder to work, because they just won't try to push through us, without having proper technique and timing.
Pulling is the second aspect of a good guard. The idea is to be able to control the distance between your opponent and yourself. You absolutely have to be able to go both directions to generate either sweeps or submissions. As the level gets better and better, this allows you to build techniques which work in opposite directions. The clearest example I can think of off the top of my head, is the armdrag to kimura sweep from the closed guard. First, you pull your partner initiating an armdrag. He reacts by pulling full force backward. At that specific moment, you let go of the armdrag and initiate a kimura sweep. You have effectively pushed and pulled to get your point across.
The last aspect of a good guard is probably the most overlooked, in my opinion, and that's Posture. I can't stress posture enough, especially since it matters for both people. If you control your opponent's posture AND can push and pull, you will upset him quite a bit because he not only has to worry about getting swept, but he can't start attacking until he gets proper posture himself. On the flip-side, if you allow your posture (especially your neck and hips) to be controlled in any way, your opponent now has the upper hand. An innocuous-looking collar grip, especially from experienced black belts, will definitely cost you five moves later, and you'll only know how important that grip was when you get to it.
So in short, I'm sorry for the long post, but essentially a good guard all comes down to being able to Push, Pull and control the Posture. The beautiful thing about it, is there are a million ways to do that : there are no limits to how this can be applied.