woahdae

One option that hasn't been mentioned - upgrading your MTB tires (or tires in general) to something supple and minimal rolling resistance. Compass has a good line of 26" tires, and there's less expensive but similarly constructed tires out there too. I can only vouch for Compass personally, but you'd be amazed what a difference tires can make, both in terms of comfort *and* effort. Unless you're riding in a perfectly smooth velodrome, wide & supple tires (emphasis on supple) will get you there with more comfort and less effort.

Also, maybe new handlebars? The VO Crazy Bar and Surly Moloko bar, or butterfly bars too probably, offer more hand positions while still being similar to flat bars.