I don't have a GPS, though I might get one in my rental car. My best preparation is to collect maps and add them to my planning document. I also want to orient myself before I go. Google maps and Google street view are my favorite resources.
For the maps, I can print them out, or I can grab screen shots and put them into my document. I do either or both, making sure I have clear diagrams for the streets around each repository: library, genealogical society, circuit court building, county office building.
I zoom in to see downtown business names. Where are restaurants and, more importantly, where can I find coffee? There's no Starbucks in downtown, but I do see a coffee shop and bakery.
Next I put Google maps into satellite view to examine the downtown area. Where are the parking lots? I look around each location carefully. I need to plan on a little bit of walking as the three downtown resources are spaced out 2-3 blocks apart.
Google Maps and Street View
Last I use street view to virtually visit each location. Since one of my friends didn't know about it, I'll assume others may also be wondering what in the world I'm talking about.
Let's take a look at the Decatur Public Library. First open Google maps and type in the address to find it. Zoom to a comfortable level to see the street names.
Click on him and hold the button down and pull him towards the map marker. Every street that can be seen in street view will turn blue. Not all streets are available in all places, but Decatur is certainly well-covered. Notice the area around the legend at bottom left where the streets are still white. Those streets are not available in street view.
The green pointer shows where the little man is going to drop. When you have it positioned where you want him, release the button. Be patient, as it takes a few seconds to bring up the images in street view.
Now he's standing in front of the library and I see what he sees. He is my eyes. I can turn around in different directions, walk down the street by clicking on arrows, visit a cross street or zoom in to see a distant building.
The photos in Google street view were taken on a particular date -- they're not real-time. This one was taken in March of 2012.
After I've visited each location in street view, I feel a little more ready for the real thing. I use this technique when booking hotels, also.
Play with street view if you haven't already done that. This tool is one of the best creations since sliced bread!