I know this is an older thread, but in case anyone come across this page I thought I would share my approach.
Google maps is your friend:
Google Maps
For those who do not know, Google Maps has some pretty great features. Rather than setting your start and end points via the address searching, I zoom in to the spot, right-click, and select "Directions from here" from the menu. Grab the green A marker and fine tune the placement. Do the same for your destination selecting "Directions to here".
Now the fun, and very powerful part. Obviously it will try and route you the quickest way. This usually involves silly freeways, and other over crowded roads-ways. Start with a nice wide view of your route. Click somewhere on the route line that was plotted for you, and drag it to a new location. A white circle will appear, that represents a new destination or via. You can continue to add and manipulate these to your hearts content. I tend to start wider, and gradually work my way into tighter zoom levels searching out the twistiest roads I can find. Be careful to ensure your marks are on the roads you intend. Also, take care to ensure enough marks to guarantee the route you want. I have noticed a limit to the marks you can place. You will recognize this when the route just starts drawing big straight lines cutting across the route. You will need to remove some, and rethink some of your marker placement. On long trips I tend to need separate maps for different legs, or days of the trip. I like lots of roads and turns, so I end up with a lot of markers.
When you have your route all laid out, click on the "Link to this page" link in the upper right corner of Google Maps. Copy the link it gives you, then navigate to this page:
Google to ITN
You can ignore the maps on this site, but notice at the very bottom a text field and a button labeled "Make ITN". Paste the Google Maps link you just copied into the field and click the button. Rather quickly a new ITN file will download to your computer.
Plug your TomTom unit into your computer or mount the SD card with a card reader device. Take the newly downloaded .itn file and place it in the itn folder at the root of the SD cards file system. This is important, and the only way they will show up on the TomTom.
Disconnect the TomTom from the computer and power it on. (This next part is from memory, as my TomTom is dead) From the device screens go into Route Planning, and select Options. You should see a button to Load Itinerary. Touch the button, and you should be presented with a list of saved itinerary files. You should see the file name of the .itn file that you copied over. Select it to load this itinerary. Your device may prompt you to choose Fastest/Shortest ect. Start with Shortest and check that the directions that the TomTom created match your intended route. If not, you may need to try either Fastest routing. If that still did not do it, go back to the Goole map link and add a couple more destination markers by dragging out a few more white dots, and rebuild the ITN file.
All in all this one of the best methods for route creation of any form I have ever used. The best part is you can do it on any platform (I write Mac software, and don't touch windows anymore). You also have the benefits of Google Maps other features like the Terrain View for checking altitudes and finding those canyon roads. Or the satellite views for checking if the road you want to take is paved or dirt (saved my ass a few times).
It is great for complex routes, and I think far better than any other GPS units provide. However, I am exploring new options now, as I am never purchase another TomTom unit.
Have fun.