Most people may not be worth talking to, but it's my job to talk to people. I'm a private English teacher and my job is to carry on conversations with my clients - often I end up doing most of the talking if they are shy at first and I need to get them comfortable before they are willing to talk with me more openly. If a lot of that talking is whining or boasting, I might scare them off and lose a client. I'm also 25 and haven't been in a relationship since I was too young to buy alcohol, and it would be nice to not die alone and all that.
I've heard of Ben Franklin's method. I also saw Demetri Martin's special "If I" where he talks in detail about how he used this method for years and failed utterly. I think this method probably wouldn't be too useful for me, as I spend the vast majority of my day either on sardine-style crowded public transportation or up in front of a room full of people (or sometimes in a cafe or something one-on-one with a private student). Also I'm a girl so I have teeny tiny pockets and I can't carry much beyond a bit of money and my apartment key in them - no room for a pen or anything like that, so I'd have to open up my backpack and get one out every time I wanted to write something down.
eerr, you're right - it's not something I normally focus on. Think about it - how often do you ask yourself the question "am I complaining right now?" It's hard to keep at the front of your mind when you have so much else to think about (am I making eye contact enough/too much, is the other person interested, should I pause for a moment, am I slouching again, and oh yeah, all the information I'm trying to convey in the conversation). I have tried writing things on my hand but I just forget to look at my hand until it gets washed off.
I've asked my flatmate to please politely point out when I'm complaining from now on, and he reluctantly agreed (as I expected, he's not too excited about taking responsibility for my problem, but I've helped him out a lot with his problems so finally he couldn't really say no). That was two days ago and so far he hasn't said anything - I think knowing that he might say something is helping me stay aware of it. Yesterday evening just before I was leaving for my last lesson of the day, I almost gave out my usual "ugh, I don't want to go to this lesson," but caught myself and kept silent. It was tough to hold it in but I managed. I was so proud! Maybe I'll manage this after all.