I've had these on the rear for over a year now with no shifting or slipping out.
Superior Automotive/Donut style coil spring booster 18-1901 at AutoZone.com
The directions say that you slide these up the coil, but I just installed it between the top spring perch and the coil. I cut the edges of the groove off one side so they would fully seat on the perch. For the price, you can't beat it.