I know I'm preaching to the choir here, but was looking for lighter gear on the Gossamer Gear site and ran across these helpful tidbits with regard to using sleeping pads (on the ground)... I was amused, so thought I would share...

Sleeping pad secrets
I realize sleeping pads is a personal thing. I know people who absolutely feel they canít sleep on anything less than a thick air mattress, and Iíve hiked with guys who didnt even bother with a pad at all. If youíre willing to experiment, you may be able to get by with very little pad indeed:

Go with foam
The evazote foam used in the Nightlight pads is incredibly light, and a great insulator. In the longer versions, itís bulkier than an air mattress, or a combo thermarest type pad, so it will take more room. The foam provides great insulation also. So try out foam. On the convoluted pads, some people like the bumps up, some say itís better bumps down, so try both ways to figure out what works best for you.
Contour your sleeping area
ONLY do this in a responsible Leave No Trace manner , where you have sand that can be smoothed back over, or duff or pine needles that can be replaced. If you choose your sleeping area well, you can create a small crater shaped for your butt, and an adjacent slight mound for your lumbar region. This will spread pressure evenly, supporting the small of your back, and will let you sleep like a baby. The most comfortable night I ever had was when we camped in an area they had been chipping the lower tree limbs. There was a thick bed of wood chips , and I got the butt crater just right. I drifted off and didnít wake up until the sun was streaming in my face!
Lumbar wad
Sometimes youíre not in an area where you can create a butt crater. In those cases, I like to wad up a small piece of unused clothing. A Driducks jacket works well. If youíre not in bear country, some food items or even trash in a double ziplock can work. You toss this into your sleeping bag, then when youíre lying down, position it in the small of your back. This serves the same purpose, supporting the small of your back and spreading out the pressure of contact with the ground. When you turn on your side, simply move the wad so it is against your side, at your waist, and it will take some pressure of the hip. If you get good at this, the results are amazing.
Tailbone pad
Since most of the pressure is concentrated on your tailbone (on your back, or hip if youíre on your side), you can cut a small foam circle, and toss it in your bag, to get a double thick pad at the pressure point, without having to carry the weight of a double thick pad for the entire pad. Just adjust it when youíre in your bag to the correct location, and youíre good to go.
Better living through chemicals
Okay, not everyone will agree with this, but a mild sleep aid can help take the edge off at night. It helps you drift to sleep, especially the first night or two when all the sounds are unfamiliar. And something like Tylenol PM can help the aches from a long hard day of hiking. Medicate responsibly.

Yet another reason(s) for why I hammock.