I use a combination of paper & GPS software. As much as it's fun to think about my route before I go, I usually have a make-it-up-as-I-go-along attitude once I'm out there.
Before a trip, it's inspirational to pull out my DeLorme Atlas & Gazetteer and Benchmark Maps Road & Recreation Atlas
paper maps and mark up my ideas with a yellow highlighter pen. For Mexico, I do not
mark up my Baja Almanac
because the darn book is irreplaceable (out of print).
I'm a collector -- and I collect (in a big Excel spreadsheet) the GPS coordinates of Points of Interest in regions I'm likely to want to visit. That is: ruins, hot springs, petroglyphs, mines, lookout towers -- in the American southwest and Baja.
Before a trip, I take the POIs that will be nearby my route, and import them into my GPS mapping software. In the US, this means converting them to GPX-format waypoints and importing them into Overland Navigator
for display on USGS topographic maps. For Mexico travel, I convert them to OziExplorer
's proprietary WPT format.
I do the same thing for nearby GPS track logs -- my old recorded tracks, and any related tracks I've found searching the web.
On the road, I run both "streets" GPS mapping software and a "topo" product, both with my POIs and old GPS track logs showing. Here are some photos and details
And, boy do I wish I could use Google Earth satellite images when out in the field!