That said, several of today's fabrics are designed to pump moisture away from the skin as rapidly as possible. Look for things made with Polartec PowerDry, which is a "bipolar" fabric, meaning its inner layer and outer layer are different. The inner layer is soft, dense, and designed to "wick" away moisture. In other words, it vacuums moisture up from your skin and carries it to the more open outer layer, where evaporation can work its magic. Mountain Hardwear's Extend Zip T ($60) is made of this stuff; so too is the simpler, short-sleeve REI OXT Zip Top ($32).
Anything with polyester is also good, as polyester doesn't absorb as much moisture as other fabrics, so it dries faster. I really like Patagonia's Silkweight Capilene, such as the Long Sleeve Crew ($34). It's an excellent base layer. And you're doing the right thing by carrying some spare base layers. But a sweaty back is one problem you'll have to endurethe pack will always ensure that your back is soaked. (I once had an old knapsack with a wool felt back pad. Like strapping on a sheep on heat.)
Still, some people just sweat more. The best you can do is control it as much as you can, and learn to live with it.