The best way I can sum it up in one sentence, is to clean up after yourself.
Try to keep the sink clean, especially after you shave. Girls tend to focus on things in the bathroom that DON'T get cleaned up, like little loose hairs, spots on the mirror, toothpaste in the sink, etc. Cleaning up these little things on a regular basis goes a long way.
Make sure you don't get into the habit of leaving towels or dirty socks laying around... put things like this in the laundry hamper right away.
Put the toilet seat down.... there's nothing quite as shocking for a girl as getting a wet rear-end in the middle of the night because you absent mindedly sat down to use the can, and the last guy in there didn't lower the seat. And make sure to wipe it off if you need to (same goes for her too, of course! Girls can be guilty of dripping on the seat too!)
Having a can of Oust or Febreeze in the bathroom is always a good idea.
Girls tend to have much more bathroom stuff - hair stuff, soap, lotion, etc, than guys do. Hopefully this doesn't bother you. If it is a problem, you two might want to work out some kind of organization of these things that works. Make sure the little trash can in the bathroom (if you have one) doesn't get too full. Some people neglect this until you can't possibly put one more used tissue in there. Ew!
I'm not saying all girls are neat and tidy, believe me, I have seen some pretty horrific all-female bathrooms. But in general, girls tend to keep the bathroom cleaner than guys left to their own. This is not a reflection on you, of course, since I dont know you... just a general statement. Only time will tell how much your cleanliness vs. hers measures out.
As far as what I have liked about sharing the bathroom with a guy... me and my ex husband had a bathroom with a big mirror and two sinks. That was nice because we each had our own space, and could stand in there together, and often had pleasant conversations, it was alot like chatting over the breakfast table. Other than that... if the relationship is more than friends, taking a shower at the same time is always nice! :D
Sorry if this is too long... I like to write :) Good luck.
my own experience