Replacing iMac with new MacBook

I am considering replacing my three year old iMac with one of the new MacBooks (if the rumor for new MacBooks is true). Since the SSD drives are much smaller I am considering an external hard drive solution as the primary storage for my documents, and only cary a limited number of docs on the MacBook.

I'd be interested to know what recommendations you have for such a set-up. I know I could simply purchase a single USB drive, but I know some have more elaborate set-ups. Thanks for your input!!