Shear in spring, if you have not kept up with it and they have become homely you may have to replace instead.

Most growers recommend twice a year: lightly in late summer/early fall to remove spent flowerheads and tidy up the plant and a closer shearing in spring when new growth emerges. This keeps the plant dense and compact and promotes more flowering stems. Just be sure not to cut back into the woody portion or below foliage as it won't regrow from that point.

It is virtually impossible to regain the tidy appearance of a lavender plant that has NOT had this type of systematic attention. They develop a lot of woody interior growth and a tendency to splay open. When that happens, it is best to replace, as bboy notes.

As a corollary, when is a good time to take cuttings? I have a bunch of lavender on my property, and most of it has been neglected past the point of repair. But I would kind of like to try propagating them, I've just never been successful with softwood cuttings of any plant before!

Try to get a shoot from the base near the soil that has a few roots, clip back and plant. I also found it easy to grow from seed, although it takes a couple of years to get a good sized plant.

I usually give mine a bit of a haircut in the fall so i can use the flower heads to make lavender oil, and then as suggested another clip in the spring...though now that we are talking about this i am thinking maybe i should replant some new ones...

I heard somewhere that if you bury an old lavender plant, leaving a lot of the top twigs exposed, these will root and form new plants. Is that true? And if so, would it work at this timne of the year? (Not that I'm too cheap to buy new plants, it just would be interesting if it worked.)

I think it would work. You would probably leave it buried all winter and then dig the plants out in the spring....?