Imagine a lightweight, synthetic layer that sits but doesn't stick to your skin. You sweat profusely but the material sucks up the sweat as much as it can, allowing the air to actually flow over your arms. Now imagine pouring a little cold water down your arm (say you have a vent on the bicep) and then you hit highway speeds. Thewater actually feels cool and better yet, the shirt is absorbing the water cooling your skin a little bit. There are a ton of options out there, synthetic, bamboo, merino, something for everyone.
You don't have to spend a lot of money, either. Try going to your local sporting good store and try inexpensive running baselayers. You can easily find a shirt and pants for ~$25 each. Just make sure they have flat seams. Raised seams will drive you crazy! And yes, bottoms are just as important as tops. Moreso if you wear leather. It will also make taking your gear on and off a breeze, no matter how much you're sweating.
Anything you can do to maximize comfort will minimize distractions. If all you can think about is how hot you are, how can you think about that car 2 lanes over about to merge erratically into your lane?