I have a probably-not-dangerous habit that I'd like to break. I've heard of people wearing a rubber band on their wrist and stinging themselves with it when they catch themselves doing something they're trying to avoid. I don't know if this method is quite severe enough for me. What are some ways I can punish myself when I catch myself "in the act", and finally break my habit?
Some people have kept a jar or cup, and required themselves to put in money (a small amount, generally no more than a dollar) each time they do the thing they're trying to stop. To be most effective, you need to designate the contents of the "sin jar" to go to something you get no benefit from -- donate to a charity, perhaps, or give to your mother-in-law.
It's the point of paying a "fine" each time that does the job, not the amount, so don't make it punitive -- but it's the consistent reinforcement that's needed, so don't let yourself slide, and don't run a tab any longer than it takes to get more change.
I'm not sure that the rubber band or a similar punishment technique is the best way to go for changing behaviour/habits (in a sensible human adult) - it's more like providing a momentaty relief/distract you from torturing your mind that you've done something bad (in your view). Instead, try to become more awear of it, rather than destracting yourself from thinking about it - for example, have a notebook and write down about it each time you catch youself doing it - when, in what circumstances, what were you doing, with whom, etc. - thus it will eventually stop being an authomatic response to whatever triggers it. Also, when you find out what triggers it, you may want to substitute your bad habit for something you are actually in favour of.