Hmm, haven't been an XO, but I've been an AXO without an XO (for all intents and purposes) above her. What I can say my take was on my job then was: to make sure the sim kept going and having fun, and the CO looked good.

That meant keeping in contact with the CO behind the scenes (for me using AIM):
-bugging him off the wall with questions (can I? should we? don't you think? how about? what if? how do you want?)
-sitting on him (aka "reminding him") to get stuff done
-making sure he told me when he wasn't going to be able to do something and offering/getting him to ask me to do it instead
-keeping up updated on what was going on in the sim and what was going on with crew members OOC that might impact the sim somehow
-getting him to stick to the regulations of IDF and to get rid of dead weight when I saw it
-bringing problem situations to the COs attention and sitting on him (oh again!) until he made a decision on a solution, even if that was to delegate to me

It also meant helping the rest of the crew:
-setting up other PCs to post
-doing very little about my own character unless another PC pulled me into something
-summarizing the mission thus far whenever someone said "I'm confused/lost" or "help!"
-standing up against the CO for a crew member when needed


Of course, the more active the CO is, the less active the XO has to be, except during the CO's vacations. But in general, just keep communicating with the CO, make sure the CO keeps communicating with you, and you should find out what you need to do to be a good XO.

-VC