Tips for getting/keeping the underhook

Hey guys I'm still workin my half guard game from the bottom and now more from the top as well. I'm looking for ways to get and maintain the underhook mostly from the half guards top and bottom. Other positions such as side control top and bottom welcome too but mostly the HG I'm lookin for. After the Ryan Hall seminar at my school today it made me think. Ryan was very analytical from every position breaking down small details like no other.
People seem to always teach the actual move but never the small details that put together the big picture. The underhooks is something I have never seen specific videos/instructionals for but I think you can make at least a long video off of this subject.

So right now from the top of HG and side control I use a crossface with my forearm and I pummel back in hard. Anything else that work well for you guys?

 

you pretty much have it with the cross face. The only other thing I can think of is gain head control by driving your forehead into the side of their head to create a gap to pummel in from. We do that in wrestling from the standing position but it works from top or bottom of half guard as well.

 

From top half guard say they have an underook on your right side. I keep my legs wide and butt low and keep my weight on my wizzer so they can't just go right to my back. I then reach to the back of the colllar to flatten them out taking away the back just that little bit more. I will then usually come around the back of the head with my left arm and then in front (to be nice dive it to the chest instead of across the throat. I then create separation with my body and re-pummel and keep my opponent flattened.