Drop the swimsuit.
As I mentioned before, swimsuits — or clothing of any other kind — are strictly forbidden in an onsen. You best be fully naked in that bathroom…
Grab a modesty cloth.
…except for your modesty cloth, a tiny little scrap that you can use to cover yourself when entering or exiting the shower area or bath. However, you shouldn’t take it into the bathwater with you.
Scrub yourself raw.
When showering at the individual slots against the walls of the bathroom, many people go beyond the simple wash-shampoo-condition-rinse routine. Though most onsen provide you with body wash and shampoo, many people bring their own products, razors, loofahs, and pumice stones. An onsen is a place to treat yourself.
—excerpted from Beginner’s guide to the Japanese onsen