From the bag of tricks...
The following tip is from Sue Luby's book Hatha Yoga:
When approaching a stretch, say a lying hamstring stretch (you are lying on your back, right thigh pulled to the chest and fixated with your right arm, left hand around your right foot, now you pull your feet while actively straightening your right leg) try the following:
Pull your lower leg a certain distance while you exhale, stop and relax, while inhaling go back half the distance you gained and relax again. Now on the next exhalation pull your lower leg again a similar distance (as your starting point is improved you will get in a deeper stretch), stop and relax and while inhaling again go back half the distance you gained, much like a ratchet. Repeat until you can no longer proceed and then stay there, and repeat the sequence once more. Surprise yourself how far you could really go. The whole process may take minutes, so don't be impatient.
It is indeed surprising how much of a difference this ratchet approach can make. There are some similarities between this technique and what Pavel T. calls pneumomuscular flexibility training (from Beyond stretching Russian...) but in my experience the former approach works far better.