Japan is the "epitome of discipline" when it comes to bathing in onsens or hot springs.
More than literally bathing and cleansing the body, the Japanese are fond of dipping in natural hot water for relaxation. It is also believed that the natural onsen water is therapeutic. The sulfur content and the minerals are good for skin and can even cure extreme sensitivity for coldness.
Before you soak in an onsen, here are the things you must remember:
1. Bathe before you enter an onsen. It is necessary to wash yourself first, while sitting in a small stools using shower/dipper and soap (and other toiletries). Make sure that you get rid all the soap suds to avoid polluting the onsen. Conserve water if possible and return everything you use in their right places.
2. Wear nothing. When you are in a public onsen, you might be quite hesitant and shy, but that's the rule. Don't worry, you have a towel to cover your parts. You can't wear slippers too, you might slip embarassingly and your towel cover might accidentally be taken-off.
3. Never rinse your towel in the onsen. As a rule, nothing can touch the onsen but your body alone. Washing anything in the onsen water would mean dirtying it. Put the towel on top of your head so you can use it again when you get-off the hotspring.
4. Don't splash nor dive. Just relax and plunge into the water with ease.
5. Don't peek through walls to see the opposite gender. Just don't! You might be kicked-off! Respect the place and enjoy your "me time".
6. Be wary when you have tattoos. Tattoos are uncommon in Japan. They don't perceive it as cool; for them, only the criminals and gangsters have tattoos. If the tattoo is small, some cover them with waterproof bandage. But if it's visible, other option is to rent a private onsen. Respect the request of the establishment.
7. Be cautious all the time. Pay attention to warnings or rules, some onsen prohibit taking photos.
Now you're ready to join the onsen-mad community! Happy bathing!