Baby wipes are your friend. My commute is 26 miles, each way, in the Texas heat. I keep my hair buzzed so I can stick my head under the faucet in bathroom and then hit my pits and junk with a baby wipe. I drive a few times a week, so on those days, I'll bring changes of clothes and lunches to the office.
If I don't feel up to making the whole ride, I jump the train and ride across town. I do take my bike, since the office is still a few miles from that end of the train line.
Anyway, I find that mindset is the biggest challenge. If it feels like this is going to take SO LONG, then I should probably just drive. When I do that, I almost always regret it, though. I've had to emotionally forgive myself for driving, if that makes sense.
In little league baseball, I had a coach who'd tell us "Come on guys! Get your head right with the ball!" That's how I describe my mindset for bicycle commuting.