(I'm actually not doing the craigslist or personal-ad thing right now, I'm more focused on job-hunting and real-life activities, I just got reminded of it and came up with a bunch of post ideas.)
1) Start off by explaining how you're extremely desirable and in-demand and can get it whenever you want it, and you're only single because you're very particular. Also, online dating is for pathetic losers, and you can't believe you're doing this because you're better than that.
2) Express your hatred of your desired gender. Talk about how horrible all your previous partners were and wish harm upon them, and lament how many times you've had to deal with "game players." Explain that most members of your preferred gender are bitches/assholes, but you're hoping against hope that someone won't be.
3) Express your hatred of your own gender. Tell prospective suitors about how all your competitors are bitches/assholes, but you're different, you swear. Describing exactly the kind of "pretty-boy popped-collar douchebag" or "dumb blonde self-centered bimbo" you're not will give you great credibility.
4) List all the things you don't want in a partner, in detail. Get really angry about it too, saying how someone without a college degree or over 30 or a different race isn't just wrong for you, they're gross and nasty. List a whole bunch of disqualifiers; ideally, things you hate should make up the majority of your ad. Don't worry about being offensive--hey, it's not your fault non-Aryans just don't do it for you! Subjective ones like "no crazy bitches" are good too, because crazy bitches will go "oh, that means me, I'm a crazy bitch, I better not reply."
Don't be afraid to list disqualifications that describe yourself perfectly--just because you're overweight and 50 doesn't mean you should consider an overweight 50-year-old partner, you're simply not attracted to people like that and can't help it.
5) List all the things you demand in a partner like you're ordering a fucking pizza. "I'm seeking an 18-24 year old slim woman of Northern European ancestry with a humanities degree who's willing to relocate and has no baggage, and extra pepperoni." Make it clear that anyone with human variations and foibles is not what you ordered, and you will be asking for your $15 back.
6) Optionally, you can describe yourself a little bit. Do this like you've never even met yourself. In the vaguest of terms say that you're "successful" and/or "attractive," that you have a "sense of humor," and if you really want to get detailed you can list some activities you enjoy, like "having fun and hanging out."
Also, if there's anything about you that most people would consider a really basic expectation, like you're not homeless or you don't hit your partners, proudly declare that achievement like it's a Nobel Fucking Prize.
7) Mention that you have a cock/tits (and how large, to the quarter-inch) and make it clear that you are expecting sex. Letting things develop naturally is "game playing," so let everyone know upfront if they aren't putting out they needn't bother.
8) Optional: Mention that you are seeking marriage, and someone should not reply unless they're pretty sure that they want to marry you and have/sire your babies, because you're done "playing games."
9) The only thing to consider when selecting a picture is "does this image file contain some part of my body in it?" Unflattering? Out-of-focus? Mugshot-like? A decade old? Ex in the photo? Myspace angle? Dingy blank wall or huge pile of computer parts and beer cans in the background? Sweatpants? Don't even look at it, just verify that it's technically a picture of you and slap it on up there! Only shallow people care about appearances.
If you don't have a photo already, set the self-timer (with autoflash) and stand against a wall like it's your goddamn DMV picture. And the DMV lady told you not to smile.
10) Optional: Include a long rant about something that's important to you, like your political or religious opinions or your hatred of American Society These Days or an alternative medical or scientific theory you're really into. True, this doesn't directly speak to your prospective dates, but you're expressing yourself and that's your First Amendment right.
11) Consider reinforcing your home so that the throngs of love-mad suitors don't beat down your door.