I'm not the original poster, and I'm about to write something gross, but I commute for long times too and I find the sweat less of an issue than I originally expected (I do live in a climate that is not overly hot and humid, though). I avoid using a backpack (in favor using panniers) so that my back isn't all sweaty, and for the rest I find that the sweat dries up quickly and doesn't leave a trace. In the worst case I take a spare t-shirt and a deodorant stick.