I don't bike much, but I run outside for exercise and as I'm not a morning person, run (haha) into some of the same problems (I did yesterday's run, er, jog, in about 95 degree heat). Besides the "slow down," and "cold shower afterward" advice that others have offered (well, and find shady routes with few uphills -- but that may not work for a commute depending on how many options exist for routing), I find it helpful to tie a (rolled up) bandana around my neck that I soaked, wrapped around ice cubes, and froze beforehand. I think you can also buy devices that are designed for this, but I find the bandana easy and have them already available. For running, I freeze plastic bottles (I use cheap versions similar to Vapur, but you can use pretty much anything -- just don't put the lid on and don't fill the water bottle all the way up before putting it, upright obviously, in the freezer) and carry them in my hands, but that won't work for biking. But freezing a water bottle or 2 for your holder and sipping from it (depending on the length of your ride and the heat, you may want to freeze one so it's full, or just half full and pour water on top before you set out). In my experience putting ice on the prominent veins in my wrists also seems to help me feel cooler, so frozen wristbands might be a good thing to try, though I've never done it.
I doubt any of this will help much with the sweat issue (well, except the cold shower if that's an option), but it may help with how you feel during/after the ride.