I'd been using foam pads for backpacking for a long time, but last summer I bought a used Thermarest through Craigslist. What an improvement. But after I used it continuously for 3 months in the Drakensberg, the Outback, and the Fiordlands, it would only hold air long enough for me to fall asleep. It would slowly deflate and somewhere around 1am I'd finally settle down to the ground, get a rock in the hip and have wake up to reinflate the pad. No good.
Got to patch dem leaks!
Step 1: Find the Holes
There are a couple ways to do this. I used my bathtub, which is probably fastest. Fill it with about 3 inches of water and then submerge sections of the pad until you find where the bubbles come up. Have a sharpie nearby so you can dab the pad dry and mark the hole. You can also sponge the pad with some soapy water to find the leaks.
Once you've marked all the leaks (mine had two), set it out in the sun to dry.