Hey, here's what I know, from personal experience and research. All of these require a high level of dedication and self control. but they work in the long run. Like all behaviour modification it requires work, and it's not gonna happen by itself.
1. Ruin stamina:
Figure out how long it usually takes you to orgasm. Then set a timer for that time, let's say 20min. (set it on your phone or open a new tab and type "timer 20min" into google) This is all the time you have to achieve orgasm today. If you fail, you have to wait until tomorrow. And no touching in between. Once you orgasm in the allotted time, you take a minute off the timer. So you have to orgasm in 19 min, 18, 17 ... Until eventually in a few months you will completely ruin your stamina.
This is like the Couch to 5k but in reverse. ^_^
2. Penis length:
Chastity cages worn for an extended period will indeed reduce your penis size, but it is temporary. It basically just compresses the tissue, but after you get out of the cage for a longer time, and have erections, it will return to the original size. Or if you do it for a long time, more then a year, there will probably be some permanent shrinkage. So, the goal is to have as few erections as possible, and have your clitty as compressed and I AM A STUPID FUCKWIT AND I DESERVE TO DIE!ed back into the body as possible, for as long as possible. That will shrink your clitty.
Getting rid of erections is essential if you want to reduce size. The general idea is to teach the body it doesn't need to be erect to orgasm. "Prone masturbation" is one way to get to it. Or if you don't like humping, you can rub your clitty through a few pairs of tight panties or by "shaking" a chastity cage until you cum. Or my favorite method a magic wand type vibrator.
This is the foolproof method http://littleredsissy.tumblr.com/post/101164275550/limp-clitty-training
it does take a while, usually about a month until you start to see initial results and about 6 months until you are unable to get hard even if you want.
There is one problem. Limpness training usually build stamina, not reduces it. So, only after you have trained yourself not to get hard you can work on reducing stamina. At least in theory.
Or you could try both at once if you want a challenge. Orgasm in a contained time, while also stoping if you get hard. I would say it could work. But I would start the sessions with a much longer time, like an hour...
4. sticking to it
Sticking to this types of training is hard for most people. Since the old way usually feels better, and is more "natural". So here are a few tips that will help.
4.1 Write it down. For some reason if you write down your goals, you are more likely to stick to it. (it sounds stupid, but do it.)
4.2 Checkmarks. Get a to-do app and set a task tor each day. And when you are done, check it off.
4.3 Social pressure. This is the best one, do it for someone else. If you have someone that you look up to, and you know they will be disappointed that you fail, you are much more likely to stick with it. If you don't have a specific person, the next best thing is to write a journal, either here or probably better, start a tumbler and do regular updates.
The first 3 weeks are the hardest, and then it gets to be a habit, and it's not that hard to continue any more. And remember, it isn't about sticking to it, as much as getting smoothy back on track if you screw it up. Learning to get back on track is infinitely more valuable.
If anyone takes my advice please report back :D