I also have a short commute, but still sweat when the weather is hot. I'll wear a t-shirt for my ride and have my work shirt in my backpack. When I get to work, I go in the bathroom and wash out the armpits and my face before changing into my work shirt. I'll also put on deodorant, which I also bring with me. Unscented baby wipes can also work well for a quick wash up.