To trim perennials use a pair of scissors sharpened they are so much better than snipping away at small stuff than those over rated long shears that we all have.  How many times with a pair of shears have you snipped off the buds that would still give us enjoyment.  Shears aren't pickie, they cut the new with the old and with a pair of scissors you can be more selective.

Cut away anything that has spent its splendor.  Cut at the next bloom or bud, right above it.  Sometimes, I clip and still loose a fresh one.  If this is the case place it in a bit of water, even without much a stem it will do one of two things if not both.  Spout roots and/or bloom. 

Little things in life eating away at your shrubs?  It's probably a disease and not a bup or slug.  There is a shrub disease called Shrub Hole Disease.  It leaves little holes in the center of your leaves, like the leaf was being eaten from the inside out.  If your plant is suffering from this, be sure to make sure it has excellant drainage and keeps its distance from other plants.  I also would cut out the problem and hope that this solves it before I would prune the shrub to just above the ground. 

If your are having trouble with lavender shrubs do not trim back into the wooded area.  If you do you might as well dig up the roots anbd throw the whole thing away.  For some reason every time I have done this major NO NO I have lost the plant.  So I am sending you my hard lessons in the hope that you will do better than me!