I have trouble when washing my girths. I cant put them in washing machine, so use soap flakes, hot water and a scrubbing brush, gets rid of most of sweat, but they still smell horrid!
Mine go in the washing machine aswell, i put old socks over the buckles and put it into 2 pillowcases, brings the girth up really well!! I do the same with headcollars, leadropes etc.
Just stick socks or buckle guards over the ends and put it in the washing machine. I've got two headcollars and leadropes in a pillowcase clanking round the machine at the moment. They come up a treat.