If you are talking about the strap itself then I'd just get a new strap. I've had my corrode enough that I ended up pulling the contact out of the strap. My most recent strap didn't show any corrosion or anything but started giving me wonky readings so I washed it, then got electrode gel which neither helped. Got a new strap and I'm back in business.

If you really just want to try and fix the contacts then maybe some fine sandpaper, or as a poster above suggested, the coarse side of a sponge. Could also use some dielectric grease every few days on the contacts to keep corrosion at a minimum but now you are starting to get around the cost of a new strap anyways.