Well, it depends on the type you are doing. It has been known for some time now that static stretching (Holding a Stretch) is ineffective for just about anything related to either injury reduction, increasing range of motion, or enhancing performance. Visit HERE for a great review of the literature on this subject. Also, what many may not understand, clinicians and patients alike are that static stretching may be doing more harm than good. Statically stretching to the point where the muscle shakes and/or hurts engages a neurological reflex called the stretch reflex, which produces the muscle contraction or shaking experienced or felt. Pain also caused by stretching can also cause muscle contraction on top of the stretch reflex as well– A Double Wammy!
So, stop statically stretching! What we know now is that PNF stretching, which involves a series of contractions and stretches, particularly in a 3-dimensional pattern or multi-planar pattern, has shown to be the most effective for immediate and lasting stretching effects as discussed in the linked article here. Yes, Yoga does work, but over a very long time, years to be honest. Athletes and most individuals, do not want to wait years or even months. PNF stretching at least two times a week will produce noticeable benefits.
I typically recommend my patients to stretch for at least 20-30 seconds followed by a 10-second contraction of the same muscle with a 20-25% effort, the cycle to be repeated 3-5 times 2x week or better, after every workout when the muscles are warm–like taffy. Taffy stretches when warm!
Also, it is not advocated to do either static or PNF stretching prior to explosive exercise as research has shown that doing so diminishes force production. Save the stretching for after the workout and warm up dynamically if you want greater performance prior.