Ideally cut back immediately after flowering. Remove all the flowering stems and goes to within an inch of last years growth. Do not cut below where you can leaves as it will not regrow from leafless stems. Failure to cut back hard enough will lead to a verry "leggy" shrub in a few years time.

The method given works well in milder counties, resulting in a more compact, and some say, a longer-lived plant. Where it is colder, however, it is beter to leave the flowerheads on until March/April, when new growth starts, and then trim them off. The old flower stems afford a little winter protection for the plant. Phil J

Good point Phil.

We leave our lavender trimming until the spring ever since we discovered that some birds find the seeds a bit tasty.