Hi Lisa, I use a very fine sand paper then clean off with a bit of thinners on a rag. You can also use the sand paper to gently clean up the friction pads in your hitch and then give then a wipe with the rag with thinners on it. I use thinners because it evaporates leaving no residue what so ever.
You can purchase a small tin of thinners from Halfords for a couple of pound and it lasts for ages providing you replace the cap after use.