A little mystery can be sexy and fun. Too much mystery, however, and you waste your time (wading through a ton of "non-matching" replies) and the time of others (who may assume or hope you're something you're not). Follow the guidelines below to make the best impression and better insure that you'll get more responses that interest you :)

Otherwise... it's important for you to cover three main issues in your posting:

WHY: Why are you posting?
Are you looking for a summer fling, a long-term relationship, a "good friend with benefits," or even just a date to a specific event? You'll want to start your post with this, so you can (hopefully quickly) weed out those who aren't interested in offering (or able to offer) what you're seeking.
WHO: What makes you who you are?
Adjectives are great (playful, assertive, upbeat)... but much more valuable when paired with examples ("For instance, I still get a kick out of goofing around with my niece's Playdoh, and similarly, I'm not the sort of guy to be shy about dressing up in a wild and goofy manner every Halloween.") If you describe yourself with enough detail, you'll have inherently already made significant progress in indicating the type partner you're looking for.
WHAT: What are you looking for in a partner?
In addition to the above description of yourself, you may still need to indicate some preferences ("Since I love to sing, I'm really looking for a guy who is also musically talented"), especially those that might not be intuitively discerned. Please remember to be polite here; since many preferences (especially regarding looks) are often perceived to be esoteric or offensive. It's better to frame even non-looks-related requests in a positive way, e.g., "No losers whose sum total of cultural exposure is Dannon yogurt" is not nice. "Seeking someone who both understands and appreciates opera" is much better.

Age is a number... An important number!

Just say it without the "-something" and without defensiveness! :)

THE NUMBER, PLEASE: Clearly state your age.
It's okay to write that you're looking for someone "twenty-something," but saying that you are twenty-something, for instance, is disrespectful to those who'd like to better understand the context of your life. Being college-aged vs. being out in the workforce for half-a-decade usually suggests different attitudes and different levels of experience in life.
DON'T BE DEFENSIVE: Be direct, and be proud of your age.
Indeed, it's not something to be ashamed of ("40something but young!"); defensively justifying your age likely accomplishes exactly the reverse of what you're trying to portray. Don't SAY you're "young at heart"; instead, describe what makes you act and feel young (preferences, activities, etc.)

Looks are skin deep, but nonetheless crucial

Have a picture ready to send, and use specific numbers in your description.
Your aim, typically, is to meet someone eventually in person, so why beat around the bush? While in an 'ideal world' we'd all like to be judged on seemingly less superficial matters; the reality is that we have to be attracted to the person we kiss. Some people have wider views of attraction than others, but let's face it -- for 99% of us, looks matter.

PICTURE YOURSELF!: Have a clear JPEG photo of yourself to swap.
Everyone has access to a scanner either at home, via friends, at work, or -- worst case scenario -- at Kinkos for a $5 scanning fee. If you don't care enough to have a picture of yourself to show someone, that says a lot about your lack of respect for the folks you're hoping to attract.
BE SPECIFIC: Be absolutely up-front about your size.
"Full figured," "big boned," and "fit" are relatively ambiguous descriptors when it comes to body size. Just be direct, and specify your height, weight, etc. If you're concerned about people not contacting you due to your specifics, consider this: are they the sort of people you wanted to hear from anyway? Save everyone some time off the bat

Don't forget other valuable details, too!

It's better to be "too" specific than not specific enough, especially when describing yourself!
Here are some other key issues that you may wish to highlight in your posting:

LOCATION: "I'm in San Francisco, and willing to date someone up to an hour away."
ETHNICITY: "I'm Hispanic, but open to dating people from all races."
RELIGION: "I'm not religious at all, and I am not comfortable around those who are."

It's all in the attitude

Be positive and don't dwell on the past
Imagine having someone come up to you in person with the following opening comment

You may be rolling your eyes at the silliness of that example, but -- no joke -- many online personal ads read like that! Bitterness is no more becoming online than offline. Perhaps one of the greatest amusing mysteries of many personal ads online is the shame confessed by the posters ("I can't believe I'm doing this..."). This implies that those reading your posting are desperate and should be ashamed, too... hardly the sort of attitude you wish to convey about those you are trying to connect with!

The reality is that hideously annoying and ugly people participate in Internet personals, as do wonderfully intelligent and attractive people. Hey, it's just like "real life," except without many of the hassles and dangers :)

In other words, there's no shame in Internet dating. Get over the assumed stigma, and jump right in, unapologetically!

You are what you type

Format your note in an easy-to-read manner and don't forget to check for typos!

Enormous run-on sentences and 80 line paragraphs are, very simply, not going to be read, resulting in the double-nasty of missed opportunities and untargeted responses. If you can, use HTML (including, perhaps, bulleted lists) to make your posting easier to scan and understand. If not, at least leave a blank line every so often to be merciful to your readers.

Spell it out!

Translation: Big Beautiful Woman (typically a significantly overweight woman) from Mountain View is in search of a relationship with a single white male or single Asian male for a long term relationship in the Bay Area. Light bondage/dominance/sadomasochism is okay. Use of marijuana is a plus.

While many (maybe even most) of the 'regulars' to personal postings may know these abbreviations, lots of newcomers may not... and do you really want to exclude them... or, perhaps worse yet, field 200 e-mailed questions about "What does xxxxx mean?" I didn't think so.

Folks, this isn't the newspaper personals where you have to pay by the word! Take the extra 2 minutes to type out stuff, please! :)

Reply thoughtfully!

RESPECT PREFERENCES: Reply only if you're realistically a match!
If someone posts a note looking for a particular type of person, don't reply unless you fit that profile. It's a waste of your time and theirs to reply with an argument or justification. In other words, if the poster asks for someone athletic and fit and your idea of exercise is lifting a Miller Lite can to your mouth, don't waste his or her time. Even when the requests may seem arbitrary (weight, height, etc.), you need to acknowledge that -- while you have a right to argue your 'case' -- you should have no expectation of receiving a reply. Just move on.
SHOW YOU READ THE POST!: Comment on relevant aspects of the poster's note.
I know, it's a vicious cycle. Men, frustrated over the slim number of replies they get, sometimes resort to efficiently spamming every female who posts an ad, whether or not they're a match, playing the law of averages ("Well, maybe I'll get SOME replies this time!") Is it surprising, then, when women don't reply to e-mails like, "Hi, you sound cool. Write me!"? Show that you've read the post! Reply with comments like, "I noticed that you're a musician, and I think that's really attractive. While it's been a while since I took guitar lessons, I have a lot of respect for musicians and would love to hear you play." This demonstrates good 'listening' skills, and also shows thoughtful interest. Sure, it'll take you a while to write people back, but your efforts will eventually be repaid.
BE RESPECTFUL: Keep it polite and clean!
Do not describe your sexual fantasies in e-mail exchanges until and unless invited to do so! Women posting ads online typically do not want to know about your (alleged) sexual prowess or exploits. Really.

Follow up respectfully!

Respond to all reasonable replies within a week, and update/delete your note as appropriate

REPLY BACK: Send a reply to all thoughtful notes, even a form letter.
Barring 'spam notes,' it seems unacceptable, IMHO, to not acknowledge e-mails. It takes no more than five minutes to gather up the e-mail addresses of the 'non-matches' and quickly bcc them a "thank you, and good luck to you" note... or even quickly cut and paste a pre-written reply for each non-match as you receive them.

Here's a sample note that should be well-received:

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"Thanks much for writing. I received many thoughtful notes such as yours, and I hope you'll understand that I simply can't reply to every person individually.

Unfortunately, while I sincerely appreciate your note, I don't feel that you and I are a strong match.

Kindly understand that I've definitely been on your side of the fence innumerable times, and I know how frustrating this process can be. But please, refrain from replying back to this note so we can both instead use our time and energy more productively. Thanks again for writing, and best of luck to you. Take care..."
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Why, who, what