Check out a nifty visualization of companies in CRSP sorted into Fama-French portfolios. This is actually what is happening within the double sort according to book-to-market and size variables.
This allows to see how much portfolios change from one year to the next. Some companies are really stable. Others, however, move around the space quite a bit.
The above animation is based on output from my own implementation of Fama-French portfolio sorts in MS SQL.
It is mostly based on SAS code from: https://sites.google.com/site/jamesicbsfinance/research/reference.
Some similar code: http://pages.stern.nyu.edu/~adesouza/out/vora_palacios_amended_justbe.sas.pdf.