This is simple, but it's anything but easy. Get in the bottom position of a front squat just before the point where your hamstrings touch your calves and hold it for as long as possible.
Be sure to do these in a power rack so you can dump the bar on the pins when you're done. Start with 95 pounds on the bar until you can hold it for at least 30 seconds, at which point you can move up to 135 pounds on the bar. From there, try to increase the duration of the holds. Anything over 30 seconds with 135 is damn respectable.
Do these at the end of your workout because your legs won't have much left afterwards. Interestingly, after doing these at the end of my lower body workouts twice a week for six weeks last summer, my front squat form felt much better and I felt a lot stronger coming out of the hole, so these have value beyond just as a challenge.