There was a study out recently in the Journal of Strength and Conditioning Research (a randomized controlled trial of hamstring stretching. Jo M Fasen, et al.) comparing different kinds of flexibility training. This is something that seems to get very little attention given how important of a part of many forms of conditioning this is. Passive stretching was found to be the most effective method, with better result than various active forms of stretching, including PNF, which has king of been the ‘it’ form of stretching lately. This is basically just yin yoga for those familiar with that practice. Long, passive poses tend to get more at the fascia and connective tissue, which can account for a lot of the resistance to a stretch when compared with just the muscle. While most yoga practices have a lot more than just flexibility as a goal, if you have a specific area that is tight (or are just not a yoga practitioner and interested in flexibility), you might try holding a stretch for 3-5 minutes each day for a while to really open it up. The best stretch to use for these longer held poses might be different from the one you would use for a more active stretch, but it’s best to find a pose that you can just release into, letting gravity take over. This can also be incredibly relaxing and restorative as well.