One of great pleasure of skier of my country is taking onsen after skiing. I will let you know some manners and medical knowledge what you should do in an onsen.

I looked an instruction poster at cheap hotel in Akita Hachimantai ski-area where we visit every winter.

http://www.sanroku-j.jp/ski/ski-1.htm

The poster which is displayed at the big bath room entrance says:

Drink enough water(not alcohol) before and after taking onsen.
Do not rinse away an onsen mineral water from your body after taking onsen.
In the beginning, take ease and slow your body into an onsen.