Trip to desert from Marrakech

We are starting to plan our September 2006 honeymoon. After 5 nights in Marrakech we want to organise a trip to the desert. At the moment we would have 4 nights, 1 stopping on the way, one at a hotel in Merzouga, 1 under the stars and 1 other on the way back. Is it easy to get transport (ie shared taxi) and possible to organise once in Marrakech or would you advise booking through a travel company before we go. Any suggestions appreciated!