onsen etiquette

In an onsen with multiple baths, how many times should one wash before going in? Is it okay to hop from one bath to another without going through the whole shower routine again?

Secondly, in a mixed gender onsen, are women allowed to wear a towel or is the full monty experience?

Thanks!

In an onsen with multiple baths, how many times should one wash before going in?

Just once at the beginning. And even then, just rinsing yourself is sufficient, unless you happen to be very dirty at that time, in which case you may want to wash yourself with a shower more thoroughly first.

Is it okay to hop from one bath to another without going through the whole shower routine again?

Yes.

Secondly, in a mixed gender onsen, are women allowed to wear a towel or is the full monty experience?

Basically, it depends on the place.

From my own experience, I think that most of the time, it is okay for women to wrap themselves. Some places even encourage that. In other places, special clothes for women and men are provided for the shared section. And in a few special cases (some natural outdoor baths in Hokkaido), it is even acceptable to wear swim suits. Yet, there might also be some places, where you could get frowned upon when entering a bath wrapped in a towel.

I wouldn't worry about it unless you used restroom in between.
Some mixed onsen may sell yuyugi (or yugi, yuamigi) that looks like an oversized moomoo. If not, use your towel(expected in mixed konyoku).
http://store.shopping.yahoo.co.jp/risingsun/yuyugi.html

Thanks very much for the quick replies.

Just realised that some onsens don't allow single travellers to stay.

Can you suggest any onsen-ryokans that have atmospheric rotemburos, are within easy reach of Tokyo and accept single female travellers?

Thanks very much again!