Don't cut it back severely. Rule of thumb is to cut it back by no more than about one-third. Lavender likes a poor soil and good drainage, so your sandy soil should be OK, however, you'll need to water it more often, because despite what the books will tell you, lavender can be quite a thirsty plant until it is mature and has been trained to drink less. I would suggest that you amend the soil by adding some compost to help keep some of the water available to your plants.

Thank you, sound advice. daisyduckworth. You are worth a zilloin to us all on this site.

Actually if you start cutting Lavender back to the earliest and newest growth you see in spring...usually down inside the plant towards the base, starting at an early age (before 3 years), they can take a more severe pruning (cutting back to even the very lowest new growth inside at the base) with not a prob. This will prevent too much woody bare stems at the crown and center...but the key is starting when they are under age 3! This works if you wish to keep more compact and extremely tidy as well.
I start cutting mine back just as soon as new growth appears, but only cut back hard as described on those that sustained a significant amount of winter dessication/die back (there are a group of 3 that I must do this too every year because of where they are positioned to winter winds, etc.), otherwise I just do a lighter pruning and just remove about 5-6 set (or more depending on die-back) of leaves back in a 'rounded' fashion to tidy up as I may have left the 2nd flush of blooming wands intact over winter, etc. Once during the season I give a second light pruning while harvesting the wands, which on my Munstead, Vera and Grosso promotes a little more growth and a 2nd flush of wands:)

Just my experience :)

Vera

Thanks vera. I have also heard that they do like poor soil, which I have here on the coast. Also I read that I should pinch off flowers the first year, is this true. My sis has just put lavendar in pots and does not want to remove her first blooms. What is the best advise?

I didn't pinch of the few wands I got the first year from my 'Munstead'...they bloomed much later than normally on 6" in plants though. The next year plants and blooms were incredible :)

Vera