The best way to get lavender in shape is to cut it back in the spring. This will promote bushier growth. I would not cut back too severely, but you can probaby shear the plants back by at least half and be fine. Just wait until you see the beginnings of new growth before you prune. Lavander does not require dividing because it's the type of plant that grows from one crown. It does not develop additional crowns like daylilies or peonies do. If you like, you can give your lavender a little compost or rotted manure, but don't overdo it because these plants thrive best in poor soil. Or, sprinkle a little, general-purpose dry, slow-release fertilizer around the plants. Osmocote is a good brand, but again, don't over fertilize.