I commute in Florida (32 roundtrip) where it is always hot and humid.

What I do is this:

Take a shower BEFORE the commute. Wait to apply deodorant.
A no-sleeve UnderArmour(or any wick material) as a base is great and you will notice how "cool" your body feels when you take it off(the bonus is that there won't be much if any sweat). Don't listen to people who tell you UnderArmour sucks, it doesn't let up. I have used it for commuting for the past year or so and it still wicks away like it was day one and barely fades, just follow cleaning/drying directions. I wear a commuter pullover on top which acts as a "be seen" article as well as something for those chill mornings.

On my way home, I put on sunscreen, reapply deodorant (if necessary) and ride. Two-three bottles of water is plenty. The great thing about riding is you can ALWAYS slow your pace and not worry about the guy behind you. Soon enough, you will notice you will sweat far less.


EDIT-Everything bluefoxicy said. Also, I moved to clipless a few months ago and have noticed these things. I even tested riding clipless and clips, switching between both and maintaining around the same speed in the same gear. I felt far less fatigued using clipless.