I own a Garmin Oregon 300 and have tried a few methods for route planning and track reviewing. The Oregon came with no software for this purpose (which I think is obscene given the cost of the unit and maps).
For this purpose I mostly use the site
bikehike.co.uk which is
very good and provides both Google and Ordance Survey maps, allowing you to plan routes that use bridleways on an OS map whilst also previewing "satellite" imagery.
This site also integrates with Garmin GPS units really well. It has the ability to download GPXX files (Garmin's extensions to GPX) or transfer the route
directly to your GPS unit if you have the Garmin communicator browser plugin installed!
The site does not store your routes for you so you will need to download the route when you have finished editing it and then upload it again should you want to change it.
Check it out!