As everyone else has stated, use a battery isolator. You'll need some very heavy cabling as well, depending on where you run the 12VDC. Isolators usually come with a circuit breaker for over current protection, but if not, add one in-line between the isolator and the extra battery. It's all very straight forward and pretty easy to do.

There are three basic ways to go on the spare battery: Regular automotive battery, Deep cycle battery, and deep cycle battery that can handle starter motor loads (Sometimes called a Marine Deep Cycle battery). The later can also handle some high-current loads like running a winch intermitently. If you anticipate adding an electric winch in the near future, the last one is a good way to go. Otherwise, go with a deep-cycle battery for your invertor. Almost all automotive batterys get trashed if you deep cycle them (there are some expensive options that do it all, but why bother)

HTH,

Tom