Your sign in doesn't indicate your gardening zone so I don't know how much cold weather you may have left. You can keep lavender tidy and promote a full appearance by doing some shaping right after its stopped blooming and the flowers are fading, drying. Cut back just far enough to remove the spent flower stems.

Do your more extensive pruning early in spring as new green growth emerges at the base of the plant - but wait until this growth begins, will depend on your climate. You may find yourself taking off as much as a third, cutting back just to the new growth that is beginning to appear.

What Morz said. Be careful not to cut too far back, since Lavender won't always sprout out well from the lowest woody parts. That's so for Lavandula dentata (French Lavender) anyway, the only type I have experience with. Not sure about other types.

:¬)

Just don't chop off the back part and you'll be good to go. :)