QuizMan is right. Once lavender gets to the leggy, woody stage there's no point in trying to bring it back - you're better advised to dig it out and start again, either with cuttings or with new rooted plants.
I'm something of a lavender fanatic, I love it, and have big swathes of it in the garden. Each autumn I give it a going over with shears, cutting it back quite short. It looks forlorn, but come the spring off it goes again.
Normally just give it a short back and sides with the hedge trimmer when I remember, usually with the last or first hedge cut, while I've got the trimmer out and connected (sometime between October and April)!