Some people may prefer to put all of their less frequently used applications on one compressed disk image. I prefer keeping just one application per disk image, since it takes less time to verify and mount the image. The compressed disk images take up far less space than the full application. In many cases, the disk image is at least 60% smaller than the full application.
When you need the application, just open the alias. The disk image is automatically mounted, and the application opens. It takes a little longer to launch the application, but it should be entirely acceptable for applications that are not used daily. Using this technique can free up a significant amount of drive space.
[robg adds: The only caveat with this method is that you may have issues during software updates -- the software updater will expect to find the applications in their standard location, in an uncompressed form. To be safe, you'd want to expand and copy all your compressed apps back to their normal locations prior to running a software update. But that could be problematic if you've used up all the space freed through the compression trick -- there won't be enough room on the drive for the expanded apps. I would recommend using this trick sparingly, and then only on the largest of rarely-used apps (GarageBand?), where you'll see the greatest space savings -- and remember to undo what you've done prior to running an upgrade. For third party apps, however, this seems like a very good solution (though you'd probably have to delete and recreate the image each time there was an update).]

