I've always had trouble stretching certain things, because I'm not flexible enough for body weight to be very useful in forcing the stretch.
E.g., If my legs are straight out in front of me, I can't sit up quite straight. That means all the most common forward bend stretches, e.g., http://www.yogajournal.com/pose/big-toe-pose http://www.yogajournal.com/pose/seated-forward-bend
that rely on my body weight to pull me forward, don't work.
One stretch technique does work somewhat - the Yoga plow position, http://www.santosha.com/hala.html
I can't go as far as in that picture, but the weight of my legs helps somewhat. But that places a lot of pressure on the part of the spine that is in my neck, which I fear to be unsafe. In addition, it is hard for me to arch my back backwards in that position, which means I am potentially creating a forward bend in the cervical spine, and potentially placing pressure on the nerves in the spine - neither supposed to be a good idea in long stretches.
So for the most part, I just gave up on this. But the muscles that are short limit my ability to do a lot of things.
But the idea of a Yoga stretch cord is great. I can use a piece of webbing (which works best, because it has a lot of surface area to spread out the force), a rope, or even a few ganged stretch bands (though that is clumsy). (I tried to order a commercially made one from Amazon, with multiple loops pre-tied - something that looked like this, https://www.amazon.com/TheraBand-Flexibility-Stretching-Including-Cheerleaders/dp/B00KDO3Z50/ref=sr_1_1?ie=UTF8&qid=1492382319&sr=8-1&keywords=yoga+stretch+band+with+loops
but cheaper - but Amazon was out of stock and cancelled the order. So I improvised.)
I loop it around my foot or feet, and pull on the other end (perhaps looped around my hand, or I can tie a loop) to bring myself forward. I can still arch my back backards a bit to prevent excessive forward spinal curvature.
I've found the band works well for a number of other stretches I find difficult.
Such a simple idea, that works so well!
My gym ran free classes yesterday, and I took Yoga. I asked the instructor about stretch bands. First she warned me not to do anything dangerous, and overstretch something. (Presumably she is worried about destabilizing a joint, and/or creating a dislocation.) Then she admitted she used them too. Her reason was different. She is SO
flexible that the ordinary poses don't help her stretch. She needs to pull on something to go any deeper.
P.S. Other people have mentioned Yoga stretch bands on this forum before. But they were new to me.