I might throw into your list of must haves a UTM grid, and hope that the map has been pre-marked with UTM grids. Some of the higher end compasses include UTM grids.

... or maybe a Lat/Lon ruler

These tools help you convert the GPS coordinate into a point on the USGS Topo map --- and vise versa.

Though a knowledgable user can get by without a UTM grid or Lat/Long ruler - or make one in the field, it is much easier and more accurate to have a simple UTM Grid. My favorite on-line source for grids and related know-how is http://www.maptools.com .

By the way, my favorite book on use of the GPS along with other navigational tools is "GPS Land Navigation by Michael Ferguson. It's seciton on GPS selection is a bit dated, but the main instrucitonal contents are very very well done and easy to follow.

That book also has my all-time favorite illustration of magnetic declination - a map of the US (sorry Frenchy) with little compass needles showing what magnetic north is across the country. It helps me visualize the direction of the declination across the country. Most other MD maps only give the +/- degrees. Silly, but I like it.