One of the deficiencies of most gps instruments is the inability to store a large number of gps routes and track logs on the instrument. Magellan has recently solved this problem by supporting the secure digital memory card on the instrument. Garmin and Lowrance to date have not responded.
One solution would be to carry a notebook so you could load additional gps data as needed.
Much of my travel is offroad on a motorcycle which isn't a friendly environment for a notebook computer. Several years ago, I found a solution that works better for me. For the last several years I have been using a freeware product called G7ToCE which is a program for the Windows CE operating system running on different versions of the Pocket PC personal digital assistant (PDA). I have heard rumors of a similar product that runs on the Palm operating system but I haven't had any experience with it.G7ToCE is available for free download at http://www.gpsinformation.org/ronh/