Tricks I've found to help ride into work without melting.
Wear a skirt- less fabric and more airflow. Guys, unless you're into kilts, I guess shorts is the best you can do, but all that extra fabric is definitely hotter. Don't get me started about skintight lycra vs flapping linen. If you can, layer- Wear a tank top or camisole on top, just enough to feel decent, and cover up with a more office appropriate shirt once you arrive.
Drink a giant glass of icewater (or in this case lemonade) as soon as you arrive. This cools your core temp down and stops your sweating much faster than anything you can do on your outside. For the short (5 mile) rides I do, I don't need to carry hydration with me any more than I would if I were walking to the T. If your ride is longer than 30-45 minutes, or if you were racing in the heat it would make sense to carry water with you. But drinking plenty of water before and after the ride seems to work fine for me.
Get a handkerchief or towel to mop your brow. Nothing makes you feel more presentable faster than being able to have a sweat free face. I keep a linen hankie in my basket so that I can wipe my face when I'm stopped at lights.
I have a giant bottle of alcohol gel (Purell) in the bathroom at work, and wipe down with it anywhere I feel especially sweaty to kill any bacteria that might cause B.O. I've never had a problem when I felt like I was smelly at the office. You might say that I would be the last to know, but if you've ever been smelly (like after a a marathon, or a multi-day camping trip), trust me, you know. If nothing else the Scientist wouldn't let me wander around stinky, and he's never complained.
And finally, but most importantly, sloooooowww it down- this is a perfect time to practice extreme slow bicycling. I picked a low gear (4th of 8) and didn't go above it the whole ride in. If I was spinning and tempted to shift up, I coasted. It added about 10 minutes to my 30 min commute, but when I arrived I was no more sweaty than an average day when the temperature is 20 degrees cooler.
On arriving home this evening
honestly I would have been hotter and sweatier (and definitely grumpier) if I'd walked 10 minutes on each end of the T ride home. At least on the bike you have a breeze.
I did bike on the Charles River path so that I could go helmetless, and in a concession to the extreme heat, I did wear my "which way to the peleton" racer style helmet on the part of my ride that's on the street.
What are your strategies for dealing with the heat?