Personally, I think it helps to meet someone beforehand.  You might looking into a shorter appointment first for an hour or two and then ask for an extended one after that.

What to expect? Well, 6 hours is a long time so some kind of meal should be included (up to a couple hours depending on the restaurant) and if you want some other social activity (show/club) then you'll want to mention that.  Without meeting the person first, it's hard to gauge how things should run and how much private time and when during the session to have it.

As for preparations, you will probably need to make the necessary arrangements for dinner/show reservations, transportation, club cover/line pass/table service, drinks, etc.  Sometimes, your provider will want to take lead on this (depends on the person). Generally, you're responsible for all expenses so it's up to you how you want to run with it.

I believe the two local portals, vegas.com and lasvegas.com, have some kind of VIP services available.  One of the better locals who keeps up with the happening places around town is Xania and you should hit her website here: http://www.thecircuitlv.com/visitors.htm
She also writes for one of the altweekies, LV Weekly, but her website is a wealth of info.

Any other questions, be more direct and ask away.

Have fun,
 Bob