Pruning Lavender

I'm a big fan of lavender and five years ago planted 4 small plants. They're obviously happy in their location because they're growing like weeds! But I don't know if I'm pruning them properly. Could you give me some pruning tips, and suggest the best time of year to give them "a haircut"?

At or after flowering is the best time, because that way you don't miss any flowers- which is what we grow lavender for, after all. You can dry your prunings and use them to repel moths!
In the spring, just as the plants show the earliest signs of new growth after winter dormancy, it is a good idea to remove old branches and dead wood, especially in the heart of the clumps where overcrowding can lead to disease problems. Selectively remove whole branches; but do not give the whole clump a haircut otherwise you will cut off all the potential sites where the flowers will come from.