Forget it. Fortunately, I never injured myself 'climbing cold' like this, but rather discovered the benefits of stretching after going to a few free yoga classes at my local climbing gym. After the sessions I would go climb, and noticed the difference right away. Suddenly I could pull moves that would usually send me chucking. High-stepping was no longer such a big deal. Difficult moves that I would have tried to power through I now had the confidence and flexibility to finesse.
But eventually I was out of free yoga sessions- surely there were some resources out there for climbing-specific stretches.
I have achieved the best results in performance from warming up with 'dynamic' stretching. Dynamic stretching is defined as smoothly moving through a full range of motion. When developed for sport-specific movements, dynamic stretching is widely considered the best way to increase blood flow and helps to reduce injury. There is a great article on dynamic stretches for climbers with detailed pictures here.
When stretching during and after a workout or climbing session, it is best to practice static stretching, which is where you hold a single position for a set amount of time (usually 30 seconds or more). Doing yoga qualifies, and Kaylee Frano, an indoor-climbing youth team coach, has written a great article with the specific stretching routine she uses for her team, as well as including a more in-depth look on what the benefits of post-exercise stretching are. Her article is here.
A common problem many people run into when they first begin stretching regularly is called 'stretch reflex', where you stretch too aggressively without any type of warm-up. At the least, stretch reflex can lead to decreased performance; at its worst, a pulled muscle. Before you do your pre-exercise stretching, try doing some jumping jacks, push-ups, sit-ups, or very easy climbing to get the blood flowing a bit. Remember to breathe during the movements and stretch slowly, deliberately, and pragmatically. Just like climbing at that next number grade, flexibility won't happen overnight!
Andy Stephen, AAI Instructor and Guide