It is very important to be sure to get any and all lubricant or rust preventative off the inside of the rings, as well as the plastic inserts themselves to prevent any slippage. No matter what type or brand scope ring you employ, it will be holding a smooth surface with another smooth surface. Even the slightest bit of oily film from your fingers can cause slippage.
I always clean the rings with isopropyl alcohol just before assembly, and I am careful not to touch the mating surface once I've cleaned them. Another thing that will help distribute the clamping force evenly is once you have everything exactly where you want it, and the crosshairs perfectly level, (this IMHO is the biggest PITA), torque the ring cap screws in a criss cross pattern, much like mechanics do when torquing down an engine cylinder head. This will also help keep the crosshairs nice and straight throughout the entire torquing process. Also while torquing everything down, be sure to keep an eye on the gap between the top and bottom half of the rings. Try to keep the gap even on each side.
One other "trick" I would mention, which I used yesterday when installing a Bushnell 3200 Elite Tactical Scope on my new Savage 110 FCP-K, is to use a couple of small line levels to get the crosshairs perfectly straight. I purchased 2 of these from Harbor Freight for just a couple of bucks each:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=66318It turned out to be the best $4.00 I could have ever spent! I used one on top of the elevation turret which has a nice flat on it, and the other I placed on the flat on the receiver after removing the bolt. This worked out much better than the "look and see" method. For me getting the crosshairs perfectly straight is one of the most challenging aspects of mounting a scope. And it seems to get more difficult as I get older. Me eyes start playing tricks on me, and I have to put the rifle down and let my eyes "readjust", otherwise you think they are straight, only to get to the range and find out they aren't. By using these 2 inexpensive levels it takes all of the guesswork out of it. I think Midway sells a similar arrangement, but this was much cheaper, and worked just as well because it takes any and all human error out of the equation. The job went really well and the rifle is now range ready. I'll try and post some pics later today. Bill T.