Clipping Lavender after it has finished flowering helps to keep it compact and less "leggy". Even as the last of the flowers fade, new growth will be appearing beneath the old foliage.
Here you can see the tall straggly flower-stalks towering above the foliage. These need to come off at a level just below the top of the leaves. The best way to do this is with a pair of hedge-shears. It only takes a moment that way. A more precise way to do it, if you have only a few plants, is to cut each stem individually with secateurs.
In this next photo you can clearly see the difference between the old and the new foliage. The old leaves are long and yellow, whereas the new leaves are small and silver-grey.
Despite your best efforts to keep them trim, it is more-or-less inevitable that after a few years your Lavender plants will develop long tough woody stems, and it is probably best to replace them.
Some varieties of Lavender self-seed quite readily, such as this "Hidcote" (below).
These are little seedlings of "Hidcote" that I recently extracted from a nearby pot of Blueberries.
As you can see, I have put the seedlings into separate 3-inch pots, in which I will grow them on, under cover in a coldframe if necessary, until they are ready to go into bigger pots next Spring.
Another way to propagate Lavender is to take cuttings. All you need to do is cut some woody shoots about six inches long, strip off their lower leaves and push them into some moist compost, and just leave them in a sheltered place until they take root - which could be anything from a couple of weeks to a couple of months.
For some unknown reason cuttings placed around the edge of a pot tend to root better than ones placed in the centre. Does anyone have a theory about why this should be?
This year I have more than doubled my stock of Lavender plants, and at no cost at all. Next year I expect to get a good display.