I use VPI's screw-down stainless/Delrin reflex clamp and periphery ring. They work great IMO - there's no better way to flatten out your records short of a vacuum platter.
All the tedious set-up steps we go through with our cartridges (VTA, VTF, azimuth, overhang, alignment, etc) assume an unwarped record. IMO everything goes right out the window if you don't use a weight of some sort and something to keep the edge of the record flat against the platter.