I do back stretches but not just before riding. I do them every morning. BUT I have noticed that they help a lot when I ride because I used to get a sore lower back after riding. My friend (massage therapist) told me if I would stretch it out every day it wouldn't tense up when I was riding. She was right. I also stretch and ice my wrists every day since carpal tunnel is sneaking up on me and riding aggrivates it.
Yoga is really great for stretching and loosening up. Yes, it was very helpful when I was doing it regularly, I haven't been doing it at all this winter and notice a huge difference. After reading your post and writing this, I'm thinking I really need to get back into the routine!
I'm 16 and stretch every morning. It builds muscle, which is awesome, and get's all the kinks out before I actually do something. I normally stretch my hips as soon as I get into the saddle (grabbing my lower leg and pulling it up as far as I can get it). It relaxes my seat and helps with my hips, which hate me.
I don't stretch, thing closet is me cracking my knuckles. And then me going and doing the push your back and it will crack sometimes if its hurting. And usually when I sit its on a vertebra and it will pop back to place. I got horrible posture walking and sitting.
I have to stretch during xcountry/track but I end up doing it all the time. It feels quite amazing afterwards and I feel all flexible too (yet no not really anymore flexible then before). Eww.. I hate cracking knuckles.. just grosses me out.. i'll crack my back occasionally when needed and my knee just cracks whenever it feels like it wants to.