What drills or pattern work do you like to do with horses that need to use their rear end more? Besides rollbacks. 

I like working squares and half halts.

The best way to teach a horse how to shift that weight to the rear is to get them to lighten up in the shoulders. A horse that is heavy on the front end tends to be stiffer through the ribcage and shoulders when asked to bend or flex, especially when counter arching, so that is where I like to start. Rollbacks work on some horses but, unless they are done correctly, they can often times create other issues... like swinging their butt around and throwing their head up in the turn. It's all about that foundation. It's way easier to catch those issues early on then it is to try to correct them later. Best of luck! 

This is going to sound crazy, but forward motion.... when these horses are going down on their front end in a turn its because they are losing forward motion in the front end and their hind feet are outrunning their front feet.

Speed up those front feet. Get them reaching and pulling with that front end. 

First do some counter arcs at the trot and really ask the horse to cross over and reach with that front end. Make sure they arent too broken at the withers. This causes their head to get so low it drags their front end down. Make sure YOU are sitting on your butt and burrying up in the saddle when you are asking them to get on theirs. Put a bit on with a good bit of lift to it (even just for slow work) to help you lift that front end and move it around. Teach them to back in circles and figure eights. Maybe their hind end isnt strong enough to "sit" on or burry up with. It could also be a coordination issue.