Core work like crazy. All around high and low reps twice a week.
A chain yoke is absolutely amazing for curing this problem also its dirt cheap to make one. If you can get up to 400 on a chain yoke you will literally sprint with 600-650 Yoke. Trust me and my Max Squat is only in the low 400s but I love the Yoke probably my best event pound for pound.
Hitting some very heavy Squat Lockouts helped me alot as well as high rep squatting.
mess around with your bar placement and especially hand placement. If you have your hands up on the cross bar your losing alot of stability compared to grabbing the sides of the Yoke and pulling in as hard as possible really locks in and activates the lats.