If you want to have the most recent version of your project in iLife preview (iMovie for example), then each time you make a change in your project, you have to save it to GB and to iLife. After you click on save in GB, and then you want to close it, there is always going to appear a message asking you if you want to save it to iLife. If you want it to be available in iLife always click "yes". When you decide on final version, share it compressed as mp3 to iTunes as well. From there you can e-mail it (it can't be too long though) to someone as e-mail attachment. But foremost you need a backup, you need to set up your time machine and backup all your files. So whenever something happens to your song or other file, you'll be able to restore it from time machine.
Hope this will help.
| The problem with this is there is no message asking if I want to save it with an iLife preview.
I had the same problem: It seems, that GrarageBand is only asking me if I want an iLife preview, when I'm closing the current project or switching to a different project. And even if you save your project with an iLife preview, iMovie still does not show the preview, unless your project resides in the "Music/GarageBand" folder in your Home-directory.
I much prefer to share my music to iTunes if I want to use them in iMovie. The iTunes Library is visible in iMovie, if it is located in "Music/iTunes". It is much less bother and I can store my GarageBand projects where I want them, not where Apple tells me to store them.