This is how I do it using a Zumo 660:
I'm using my Firefox browser which has a GMapToGPX link on the tool bar (http://www.elsewhere.org/journal/gmaptogpx/
1: I plot out the trip using Google Maps.
2: Once I have the route mapped out on the way I want it to be - on Google Maps, I click the GMapToGPX link on my toolbar which then generates a GPX file.
3: I click the "Full" tab which makes the GPX file much longer as it displays each and every direction change along the route.
4: I select ALL and Copy the contents of the GPX file to Notepad so I can save it as a text type document..
5: Inside the GPX file I change it's NAME to what I want it to be named (from the default name it was given when the GPX file was generated in Step 2 above). That line is near the top of the file between the "name" "/name" brackets.
6: I save this GPX file to my hard drive as an appropriately named .gpx file. (Not a .txt file... but a .gpx file.)
7: I then go to my MapSource program and open the .gpx file I just saved in Step 6 above.
8: Once that .gpx file is open in MapSource I transfer it to my Zumo660 using the "Transfer" button.
9: Last step is to disconnect my Zumo660 from my laptop... fire it up and import the route so it will be one of my "where to" choices.
I just mapped out a 19 day trip - 5500+ miles from Panama City, FL to Billings, MT this way. It consisted of 8 days getting to Billings... Two days in Billings... and then 9 days back to Panama city, FL. A total of 17 different routes. Using GPSVisualizer - I was able to generate a Google Earth .kmz file which can be used to visualize all of the routes either in Google Earth or in Google Maps.
Here is my kmz file for the entire trip: http://hilsheimer.org/images/versys/...m_Billings.kmz
You can download that file and open it into Google Earth, or you can copy that URL (by right clicking on it) and put it into the Google Maps search box and it should display my proposed July routes.
Hopefully, this link will open up Google Maps with my kmz already loaded... displaying all of my 17 routes (days) that are planned for July - http://g.co/maps/wnyqu
I planned all 17 routes using Google Maps... Converted those maps into GPX files using GMapToGPX... then used my MapSource program to load all 17 maps into my Garmin Zumo660.