For DR presumably its just practice, practice, practice. As a passenger, I have been asked to pick random points on a map, getting the pilot to do emergency diverts to X, or Y. Perhaps you could try that? Or spend some time with a map on the ground, dong a theoretical flight with diversions but with real wind. If you find planning a nav difficult, talk to an instructor, or get an instructor to watch you try and plan and they can perhaps find ways to improve your technique.
For radio nav, I guess you could use some software to simulate the radio environment (be that a flight sim (control it using the autopilot if you are just doing nav practice) or one of the dedicated packages).
To practice DR and radio nav in the air, why not get the GPS to record your trip rather than use it for nav purposes, then download and analyse your performance once on the ground? This will let you debrief yourself.