First and foremost, this ad, which "works" - that is, helps you sell, buy, find, and so on. Don't hide your goal and clearly specify what you want using this ad to do so. View the newspaper, and there, quite often there are ads that can be understood in two ways. For example, under "appliances": Refrigerator, inexpensive phone". Buy cheap or sell cheap?
Be informative and be sure to include all the fundamental information for you. If you wrote "Buy a car b/y", but I agree only on foreign cars, and not older than five years, but still agree to spend a certain amount – most likely, a fair amount of calls will be "off the books". Yes, the ad should be concise. But not at the expense of substance.
Evaluate the situation from the point of view of the reader (customer, competitor, customer etc.). Try to put yourself in his place. If you want to buy a used sofa that you will be interested in in the first place? Dimensions, condition, color of the upholstery? That's about it and write, but not about that "he faithfully served for three years and gave a lot of pleasant moments". If you place an ad on the Internet, and it is technically possible to attach a picture, do it.
Not amalfivia about essential details. Looking to recruit a Secretary, and a salary offer below the market? And write. Yes, calls will be less. But an effective ad is not the number of calls is the result. And those who agree only on a high-paying job, still you end up not going. But you have to waste time in futile talks and raking three hundred summary.
By the way, what is your offer better than the dozens of similar? Why, for example, for photographing the wedding need to invite you? Oh, you are the winner of international competitions? Or the opposite – just starting to practice and ready to work "for food" in the portfolio? And write. And be sure to highlight (font size, bold – whatever).
Don't forget to provide contact information. Moreover, to specify not only phones, but also allowed time for calls. People have very different ideas about how much decent, for example, to call a stranger on Sunday morning.