squats leg curls and there is a machine at my gym in which pads go on the inside of your thighs and you squeeze it together. Works really well. Also, medicine ball in between legs and squeeze. This willl help with triangle and lockdown along w armbars ect...
Use the big inflateable exercise ball ..leg curl and squeeze on that thing for a long as you can. It will build your squeezing endurance for RG pretty quick. I used to do at least 4 rounds at 1 min. per round. Hope it helps