I get it from halfguard the most. But Marcelo Garcia has an awesome entry into X-guard from standing. The major point is to literally pull yourself into it, not just fall and try to get under him.
Let's say you have sleeve control with your left arm. Take your left foot and place it in the crook of his kneed. As you lower yourself, use this hook to pull yourself under him. What's most important is that you keep a hold of his right sleeve (the one in your left grip) because keeping that tight will bring him off balance and render him unable to step away. Your right arm now goes under his left leg and you hold it like doing a bicep curl. Meanwhile, don't go straight for X-guard just yet. Your right leg is still not doing much, so place it right next your left, in the crook of his knee, facing the same as your left. So right now, your left hand is controlling the sleeve and pulling him down, right arm is controlling the left leg, and both of your feet are hooked into his right knee. Now release the sleeve in your left hand and grab his ankle, then you can change your right hook to the proper X-guard. Chances are you won't need to because he'll be so off balance, that you can just keep your feet pointed in the same direction.
