Around here, it's tough to keep paint wet. On average, the temperature tops 100 degrees more than 100 days every year. Without a wet palette, paint doesn't stay workable for more than a couple minutes.
I've used this palette for years:
Essentially, it's just a plastic tray, a soaking wet sponge and a layer of special paper (actually it's a sheet of permeable acrylic). When the sponge is wet, water passes slowly through the paper and keeps the surface slightly wet. Even here, in the middle of the desert, paint stays workable for at least 15 minutes (or longer if it's thinned with enough water).
The tray and the sponge are dirt simple. If you don't want to spend $15 on a "real" wet palette, you could make your own out of a soap dish and a kitchen sponge. But I haven't discovered a cheap replacement for the acrylic sheets (although at $7.50 for 30 12"x16" sheets, they're not too expensive). Unlike sheets of paper, they lay flat when they're wet and they don't fall apart as you mix your paint. If you're careful, they're a great investment. And don't forget, once you use up one side, turn the sheet over and use the other (Just wait until the paint on the top side dries. Otherwise you'll paint your sponge).
I find they work best if I give them a few minutes to soak up water for a few minutes before I start painting. What I usually do is put the paper underneath the sponge to start and cover them both with water. 5-10 minutes later, I drain off the extra water, put the paper on top of the sponge and blot it gently with a paper towel. The blotting is very important. If you skip it, the surface will be too wet and your paint will spread all over the place when you put it on the palette. Those colorful stains on the sponge in the picture above occurred when I forgot to blot my palette.
After that, you're ready to go. If your weather is exceptionally hot and dry, check your sponge every couple hours and make sure it's sopping wet. If it isn't, lift up the paper and add some more water to the sponge.