Check out the great amount of mobile antenna information on the
K0GB.COM web site.
As I see it it kind of comes down to two areas. One is the case where you're going to be camping somewhere for a week and can get something strung up and the other is where you might want to stop for lunch or a quick overnight stop and need a quick and dirty way of getting something on the air.
One thing to look at is an end fed which can be pretty easy to set up and get running.
Lots of luck and don't get discouraged after reading through k0gb's reams of information.