Stay in a Riad!
We stayed at Nejma Lounge. Owned by a European couple (like most Riads), but run by a really nice college student named Abdel. It's located very close to the (west?) gate, so finding a taxi is really easy (and cheap!) You can also walk to the BIG square in 10-15 minutes.
They offer pickup at the airport for an additional charge, which, IMHO is completely worth it to not deal with getting scammed by the other cabbies.
If you arrive in town at night it might seem a little sketchy if you've never spent time in an old city like Marrakesh, but don't worry you'll be fine :)