The advice given is that you should always remove any imprint of the mount from your windscreen and take your GPS with you. There are literally hundreds of carry cases available to fit in a pocket. All you need to do is search for your model eg Garmin Nuvi 3790T. They generally cost less than $20.
On the shopping sites there is a carrying case for the 5" models "Garmin 5-Inch Carrying Case" that works very well and is well constructed. I have been using it for a while. It is a snug fit.
A good mount that does not leave residue is the friction dashboard mount, a new style beanbag mount. Highly recommended.
On the subject of leaving marks on the windscreen, I use one of Garmin's Universal Friction Mounts. Although it takes up more room for storage, it doesen't leave any marks and is very stable.
I just spotted a cheap, Chinese, padded hard case with free screen protector and stylus for under $10. It has room inside for storage of accessories and looks very smart. It even has the Garmin name on it.
I use a Garmin leather/suede lined slipcase for my 3790T. It's very neat and takes up no more room in my bag than my iPhone.
Officeworks was selling their Nuvi Cases that fit the 1300 rang for around $2 on clearance. I bought one of these and a $2 Neoprene pencil case to hold the mount, cables and manuals.
When I'm not using the GPS it all goes in the pencil case and then into the backpack.
Best buy carries some interesting equipment for attaching the garmin to the dash. They even have a service to permanently install it into the car. But that means using it in only one vehicle.