While having the strength to support yourself in extreme ranges of motion is a major part of flexibility, a muscle's level of tonic contraction is the biggest limit on most people's flexibility. Static stretching does reduce tonic contraction temporarily and to a small degree, but it is not very effective at this. The best way to reduce a muscle's tonic contraction is through compressive techniques, such as body tempering and variants of myofacial release. A combination of stretching and compression actually works best of all (this is similar to the active release therapeutic modality).

As an anecdote, doing an hour of yoga every day for three months didn't do that much for my hip and shoulder mobility, but an hour of body tempering by rolling on a PVC pipe while watching TV (while doing absolutely no stretching at all) produced massive improvement.