There’s a great video up at ApertureExpert on how to do this in detail, but this is the quick and dirty version.
For the “on the road” library, work in Managed mode, where the photos are stored in the library. It keeps everything in once place, but for the main computer with 30,000 photos and growing, I need to work Referenced so that I can keep Aperture file sizes manageable.
Import from Laptop to Desktop
So, after importing and editing on the Macbook (working managed): File->Export->Project as New Library.
Then, on the base machine, File->Relocate Original…
Export from Desktop to Laptop
Select what you want, then File->Export->Items as new Library
Select Copy Originals into exported Library – use the originals, and copy previews into exported library. You can just export the previews if you don’t want to edit the images, just add keywords etc.
When you bring them back, use the merge option to merge back to existing project folders.