#1 tip! Do NOT try to come off as a know it all....
Any good admin doing an interview can and will try to trip you up. (I know... I do it ;-)
Be honest, it will impress an interviewer more that you know where to find a solution to a problem than actually knowing it.
This goes for any interview.
Dress to impress, be on time! Spell check spell check spell check! (If you send me a resume with misspelled words... I wont read any further), do your homework... know a little about the company, their business, their customers, their mission statement.
When I interview I look at it this way... They have already decided that I could work for them; they have to pass my tests now. Go in with questions and interview them (preferably written down).
As for common questions, people vary so will the questions, some go all tech, others go personal, you need to have good interviewing skills it’s that simple. Practice; if you know anyone who does interviews ask them to do some mock interviews.