Lavenders tend to get leggy and messy-looking over time, and just need to get replaced every few years.  That said, you can delay the inevitable by pruning lavender (Lavandula angustifolia) lightly after bloom, while deferring moderate pruning till spring.  The trick with the spring pruning is to wait until you see the new leaves and twigs sprouting from the older wood--and don't prune any lower than the new green growth, or you will kill the plant.

By the way, when the plant just needs to be replaced, remember that you can easily propagate the new replacement from cuttings taken from the older plant.  You can do this yourself (semi-skilled humans have been growing lavender from cuttings for thousands of years).    In midsummer (not now!), fill one or more old flower pots with damp vermiculite (or potting soil mixed with about 50% perlite).  Cut off twigs of new growth long enough to have 4 or 5 nodes.  With sharp scissors, cut the leaves off of the lower nodes, leaving two nodes with leaves.  Push the twigs into the damp vermiculite immediately after cutting--timing is very important, and you do not want the cut twigs to dry out.  Using a rooting hormone, available from any garden center, will improve your yield, but you will get a large percentage of viable cuttings even without it.  Place the pots where they will not get direct sun, and keep the soil moist.  New Roots should be forming in two or three weeks.  Tug very gently on the stems.  If they come out clean, like a toothpick, they are dead; if you feel a slight resistance, that is the new roots.  If you started them in vermiculite, transplant them into pots with potting soil when the roots are about an inch long.  Continue watering regularly, but do not over water.  Plant out into the garden in late summer or fall.  Final tip:  starts lots of cuttings.  That way you can sacrifice a few along the way to see how things are progressing.  And you end up with extras to give away to friends!