For the sake of argument we will assume that you are going to be putting your personal ad on craigslist or somewhere else online, would you really want someone who was reading the newspaper personal ads in this day and age? When reading these steps you may note that some are mutually exclusive, normally I would consider that problematic, but not today.
The easiest way to scare men off with your ad is simply not to include a photo. The majority of men are going to pass this by every single time, particularly the men who have standards in a woman beyond having a pulse. You know who responds to ads without pictures? I'll tell you who, men who are so terribly desperate that they reply to every single advertisement posted in hopes of getting a response.
Instead of posting a picture of yourself post a picture of a flower or a cartoon character or a kitten. This not only tells prospective men that you are unattractive and insecure it also lets them know that you are flaky and play mind games.
If you do actually post a picture of yourself then make sure it is low resolution and cropped in such a way that your figure is completely hidden. That way you can scare away the men that assume that you are heavier and uglier than you actually are, while simultaneously setting someone else up for disappointment or yourself up for an embarrassing situation. It is better just to let them know what you are offering up front, some guys will deal and some won't.
If all you have are nice high resolution photos then be sure to pick one that also includes your best friend (isn't she pretty). Statistically speaking your best friend is probably better looking than you are, and if she isn't then she just detracts from your ad and thus helps scare men off either way. You can scare off a few extra men by not indicating which one is you.
I am going to go against some of the traditional wisdom here that would say that you should be extremely detailed about yourself in order to get the best responses. In fact you should only be overly detailed if you want to scare away men, once you get past a basic level of information you are simply providing a potential mate with more criteria that they can filter you out using. Short or long you should however use as many text message abbreviations in your description as possible in order to scare away the maximum amount of men. Your love for country music or rap might not end up being more than a tiny quirk in your future relationship with that certain someone, but mentioning it in your ad could easily scare them away. If you really want to make the men run consider mentioning how you basically just listen to Billy Ray Cyrus and Bone Thugs in Harmony all day long at maximum volume.
If you have children then be sure to mention how "they are your whole world" and how "nothing comes before them", but how they "have a daddy" and you "aren't looking for a new daddy for them". If you can only somehow mention that you don't get child support for them then you can probably manage to scare off everyone in one single section.
Who you are looking for.
Now we are getting into a real gold mine here, nothing beside that picture is going to scare away more men than the section where you list your own personal laundry list of requirements. Every single thing you put in this section will scare away men, most women list enough requirements that it is mathematically improbable that anymore meeting their requirements would ever even read the advertisement, you should do that, and more.
Be sure and ask for a tall man, the bigger the height requirement the better, best to list something at 6'2" or above, since that easily scares off 95 percent of the men reading your ad, since less than five percent of men are that tall. You should certainly require an athletic build as well, to scare off any fat or underweight men reading your advertisement. In general you just can't go into enough detail in what you want from a man's looks or body. The more you require, the more men you will scare off. If you really want to scare off some men then be sure to mention something about penis size, that way you won't have to deal with any of those short dick men answering your ad, or any of the average men either, or anyone who doesn't have an absolute whopper. While rounding out this section you should also specify a hair color, eye color and race as well, for maximum results the racial choice should be different than your own.
Finally, you can also scare off lots of men by forgetting to include your age, location or race (assuming you left out a picture here, since you are trying to scare them off).
What if you don't want to scare them off?
If you would actually like to get some good responses to your advertisement then there are a few things to look at. First and foremost you should include a picture, and it should be the best recent picture you can come up with and it should be of you and only you and it should not be carefully cropped in order to hide your figure. Believe me, would you rather have guys filter you based on your figure before you ever even know they exist, or would you rather have them do it in person while you are standing right there? Plus there is the minor little detail that many men will assume something far worse from that cropped shot than whatever your reality is.
I am going to go against some other opinions and simply state that I don't really care what sort of activities a woman might claim to be interested in on her personal ad. She is probably lying anyway, and most of life doesn't consist of that anyway. Say something interesting, minor and true. Don't tell me that you like sports and restaurants and walking on the beach. it just all ends up sounding exactly the same as every other ad. Tell me "Hi, I am Resa and I love muffins and I am looking for a boy who also likes muffins." If you have a kid you can even do this variation "Hi, I am Gabby and I like Muffins, my daughter Cindy likes muffins too. We want to meet a boy who likes muffins." Just tell me something fun or interesting, I don't even really care what it is. As far as children go they are a lot less scary when mentioned in a fun way than in an intense way.
As far as what you want out of a man, honestly the less you say here the better, if you have a fun ad and a good picture than you should have many people to pick from, so there isn't any good reason to write a huge list of requirements. Honestly we all know that you are going to filter them by photos and age first anyway!