Well, there's a difference between asking a pestering. Obviously, don't ask for admin the second you join. If the server has a ranking system, go up the ranks til you can't get any higher. Make sure you know the owner and he trusts you.
The most difficult thing for an owner is having to deal with the server, so your job is to administrate the server (See what I did thar)(Pun wasn't actually intended) and make his/her life a lot easier.