What do you do if a a customer has a Ferrari 599 and does not want to "drop" it off? I ran into this when I was in FL.
I had a shop this summer and noticed my higher end clients went away. I also noticed that this winter, I would be paying rent while the shop was closed down due to the weather.
How did you overcome these things?
In the case of a high end car/client, I think the best policy is to be totally honest and straightforward with them... the client has a very valuable car, they always want the BEST work and I can do better work in my shop. That's the simple truth.
Unless, the client has a very well lit, spacious garage with drained floors... and that's almost never the case, unless you're David/Street Dreams and work for some of his clients.

I know I'm not Todd, or David or Clark or any of the other great detailers on this site, but I aspire to that level... or, if possible, better. I want to give myself and my client
EVERY possible advantage when I do their car.
My experience has been such that I find an adequate place to detail, in a mobile situation, is pretty rare. There's almost always space issues, climate issues or lighting issues and usually all the above. The best mobile experience I've had recently was in a very spacious garage but the light was
terrible... it was really hard to see in there; had to put halogens everywhere and it's hard to do the work efficiently when you're also constantly fixing the logistics. Plus the place wasn't heated and the temps were cold... hard to do good work in that kind of environment. Space heaters helped but once again... compromise.
Security's always an issue and without getting a man-eating guard dog, I've done everything I can to secure my shop. First of all it's in a nice area of town and not in an industrial area... so it's "under the radar." The building has no windows, the garage doors have locks on the motors and deadbolts. The entrance door is a metal fire door with a lock and a deadbolt, alarm system is by ADT, the outside of the building is lit by sodium vapor lights on motion sensors... and I live five minutes away from it. All that helps me when I point these things out to potential clients.
As far as the winter doldrums go... I just save up some money for the "winter slowdown" and since I've planned ahead, I'm ok with it. That way any work done in the winter months is "bonus" money for me.