Sage being an herb, it’s easy to forget that it’s also a shrub and as it gets older needs regular pruning. (Especially if you only ever harvest a few leaves here and there and never cut off great swathes for making sage tea.)
My Mom had a volunteer sage growing between the paving stones of her patio (where it loved the drainage) which eventually developed an inch thick trunk, with wonderful slightly peeling bark, that trailed across the patio looking very ‘tumbling tumbleweed’ – all because my Mom didn’t have the heart to cut the thing back hard in the spring. Unfortunately it succumbed when the patio was replaced by a deck, but that’s another story. (Below is the before of a nice rambly sage – just not as rambly as my Mom’s.)
I generally cut sage back fairly hard in Spring, once it’s a few years old and the stems have started to get woody. The growth you cut off can always be dried for tea or such. Just make sure that if there are not healthy clumps of leaves where you cut back to; that there is at least a hint of a green shoot peeking out through the bark. As long as there are these hints of green, there is a decent chance that new growth will emerge from that spot. 
Also, of course, I take out any dead or damaged stems. (Here’s the after of the above rambly sage.)
These rules also apply equally to lavender, another herb/shrub that tends to get very woody and leggy if you aren’t careful. When it’s young I usually just give it a bit of a hair cut, but as it’s get’s older the hair cuts need to get more severe. I know that elsewhere I have railed against leaving stubs, but with lavender I find it easiest to just gather up all the stems and cut across. Stubs are minimal and it gives the shrub that nice rounded look. I know it always seems a crime cutting off all those young grey-green shoots, but at least your hands will smell great afterwards. Just don’t cut off all the new growth. You’re going for a haircut, not a shave.
pruningdiaries 