What is the best way to store stock in the freezer?
When I want to freeze portions of almost anything liquid (including stocks and sauces), I use a covered ice cube tray like OXO Good Grips.  Just spoon it into the tray and throw it in the freezer.  Easy!  Keeps very well, and makes it very easy to portion out later.

The OXO product is my favourite, but it's not too hard to find generic substitutes at any superstore.  Try to find one with a soft bottom so you don't struggle for 5 minutes trying to get the cubes out and then scraping the sides for all the bits you left behind.