After our trip to Mojave desert it was a lot of questions about APRS system. I will not re-invent the wheel. There are a lot of info about APRS in the internet.
The good start point here: APRS

my current position here: W6BVB-7

And yes, it is very useful for off-road application!

Thanks for the links.

Now I just need to figure out how to set it up...

Vlad, I look forward to checking out your position on the upcoming Death Valley trip. Have a great time!

Basic questions -- Can I assume you have control over turning the APRS on and off? Can it be set up so that it only puts out your position when you hit a button? And if that's the case will the map show your last known position, or will it not show any position when the APRS is off.

Thanks!

Yes, the transmission of APRS packets can be controlled by PTT or automatically by setting certain period of time between transmissions. It is up to APRS station owner how and when to transmit. The map will show only positions, which received by APRS network stations. Some times you are out of range ground repeaters but there are few APRS satellites, which able to pickup your transmission and re-transmit it to the network. This is very handy if you are in very remote area, where cell phones are useless.
In additional to position report, APRS protocol support sending short text messages between stations and to the regular e-mail.
Check this site: www.openaprs.net It provide a lot of useful tools like multiple stations tracking and ability to send message to the stations.
73!