I'm another convert to stretching.
last year was a tale of stiff lower back, sciatica, thigh strain and pulled back muscle which had a massive impact on cycling. for the last four months I've been stretching and strengthening using elements of Pilates and yoga and have not only noticed far less pain but also greater ease in climbing whilst sitting at this stage of the 'season' for example.
I have an experienced Pilates/yoga instructor who's a qualified physio and she reckons the root of my problem is extremely short/tight hamstrings due to never travelling anywhere except by bike for several years.
I've also worked on the piriformis muscle, something else which cycling unbalances, using a couple of excercises.
@Fnaar , you can also work this by sitting on a tennis ball and massaging it into your backside. it's a bit painful at first but gives so much relief you'll wonder why you never tried it years ago.
cycling home from an hour-long class my legs feel as light as feathers as tho I could climb ventoux on my single speed. it's quite exhilarating.