Try making a starter (methods are in the websites listed below), and instead of pitching liquid yeast, pitch the yeast sediment from two bottles of coopers or two bottles of chimay (or one longneck of either). You might want to start off with a small starter at first, then make another slightly larger starter and pitch the contents of the first starter into that (or just top up the first one with more extract after the first primary fermentation).
I personally use a sanitised 2litre coke bottle for my starters, with a bung and airlock ontop of it.
One thing to watch out for with many of the Belgian beers is their age. Culturing is only really succesful with a reasonably fresh bottle, and since Belgian beers do not exactly fly out the door of most bottle shops you will generally find that they aren't too fresh (I recently purchased a bottle of Cassis Lambic that was over three years in the bottle - bloody marvelous drop, but useless for yeast culturing).
For Coopers, I got this method off BJ's Homebrew website:
Yeast starter (3 - 5 days before brewing)
Boil 100 grams of Light Malt Extract in half a litre of water.
Chill to 25 degrees and add the sediment from 2 or more stubbies of Coopers (yum!)
Transfer to a sterilised container, fitted with an airlock.
Wait for 2 - 3 days (BE PATIENT!) If yeast is viable it will take off and put a good 2 - 3 head on the half litre starter.
When starter is highly active, it's time to brew
I haven't tried this but it sounds consistent with other starter methods.
There is a good article in the archives by Chris White of White labs .
This article details everthing you need to know about culturing yeast lees from commercial beers.
Coopers is a nice easy one to culture.
I cutured some from the strong ale they make.
Other beers can be very difficult without plating out and growing individual colonies etc.
My opinion is, buy a known liquid yeast that will probably perform better than a old bottle yeast anyway (more viable cells free from mutations etc). Then keep the strain going. (storing and reusing)
If you want to experiment then mix some of the strains you have (a common belgian tech).