Am I right in assuming the Retina MBP has an mSATA SSD inside? If I am wrong, why not simply buy a larger mechanical 2.5" or 1.8" internal drive? Yes, performances will decrease, but it may be worth considering.
So what do you folks do? Where do you store the data on the computers that can't hold it all?
I don't. I keep my data in my home server or my office file server, and only a minor part of it is on my laptop. I currently have an 80GB SSD in my work laptop and it is enough. Clearly, that many not work for everybody.