When I have someone in my half guard and they reach across to the far side of my body, sit on the floor and hold my knee to free their leg and go to side control? 
You mean when they hip switch?
Goatfury has an excellent tutorial on a sweep.
Grab pantleg with right arm, insert butterfly hook, hide left arm around their neck, pull pant leg grip, use butterfly hook and and around neck to sweep. Horrible explanation, but his video is great.
The other option is to feed their gi under their leg, use a hook and come up, but this is probably more for when they backstep.
I suck at explaining things.
Go to BJJrevolution or it might be in his 80+ tutorials on 1 page bit. Check that thread.
Other option may be to use the Marcelo stiff arm on their near shoulder to hip away as they free their leg, and reguard.
carcaju -I love that move. Simple and effective
Other option may be to use the Marcelo stiff arm on their near shoulder to hip away as they free their leg, and reguard.
There is also a backtake option too, but requires you to really push your opponent as far down your body towarsds your legs as possible.
Just tttd the thread for you . 80 plus techniques.
carcaju -Ok thanks guys what is ttt?
Just tttd the thread for you . 80 plus techniques.
It's called "New project organizing 80 plus tutorials in one spot " by goatfury
1) Early - The largest priority on HG bottom is stopping their weight from crossing your centerline. You should be keeping his head to your right (if his right leg is the trapped leg). This will prevent the entire situation.
2) Middle - As soon as he turns his hip, you need a left butterfly hook under his knee. When he raises his right knee, you must be ready to help him raise it even higher. This will set up a variety of sweeps.
3) Late - at the moment he frees his leg, turn into him and double leg him onto his back.
Focus on the first two.