There are lots of ways to plan a trip and everyone has an opinion on the best way. I'm not here to discuss the best way, just different ways of doing it. I'm sure there are lots of places to create tracks including Google Earth, but here are some that I feel are some of the best for doing so. It's always great to have real maps handy when planning and not just relying on the internet for everything
Use these links at your own risk, use good judgement, ATGATT, carry paper maps, and be safe.
If you wanna just do it old school, by hand, you can do it with Mapsource and save it as a .gpx or .gdb file if you are going to put it on your Garmin device. You probably need Topo or City Navigator installed to make this original trip planning method effective. Once created, easy download to your Garmin device.
Riding Social was created by the boys at Butler Maps. They have an app for your phone (Android and IOS) where your tracks online get synced with your mobile device. You create your trip online in a very intuitive manner. They have extra features that cost a little money. The only downside currently, is you can't have a 'collection' of tracks for a single ride. You need to plan your entire trip as 1 single track. This is a work in progress and it's actually somewhat easy to get around. Once your trip is done, you can import it into MapSource, and just cut it into the pieces you need. Currently this is my method of track creation and chop it up with MapSource.
This is another very cool trip planner which is easy to use but I have very little experience with. I'm currently spending some time with this to get well versed in its features and trip creation abilities.
Map My Ride:
A map planning website suggested by another user that I don't have any experience with, but worth a look.
Dual Sport Maps:
Currently in a state of limbo, and hopefully Josh has time to work on it this year. Dual Sport Maps lets you create an account, upload & download tracks. It is currently in limbo as Google changed from the v2 API to v3 which killed some features. You cannot create or edit tracks on the site currently, but hopefully that feature gets fixed. It's just a huge time consuming project and I know Josh is very busy with his MotoBrain project.
If you have gps track file and need to convert it to a different file type, here are some good resources.
Converting file types:
If others have methods of trip creation, I will gladly add them here.