The easiest way is to saturate your skin with something greasy, like Vaseline, leading up to the fight. Your skin will look and feel normal, but the moment you start to get a good sweat on, your perspiration will be a Vaseline/sweat mixture, making you much harder to get a hold of.
What if somone accuses you of greasing during the fight and the referee checks you? I think you can tell the difference between sweat and a mixture of vaseline/sweat if you're experienced.