You need to write an ad. It doesn't matter what: maybe you want to sell (or buy) something, offer your services, hire a specialist, exchange an apartment. In any case, in order for your ad to be effective, it must meet a number of requirements.
First of all, this is an ad that "works" - that is, it helps you sell, buy, find, and so on. Do not hide your purpose and be very clear about what you want to do with this announcement. Look at the newspapers, there are often advertisements that can be understood in two ways. For example, in the section "Household appliances": "Refrigerator, inexpensive, telephone." Will I buy or sell inexpensively?
Be informative and be sure to include all the information that is important to you. If you wrote “I will buy a used car”, but agree only to foreign cars, and not older than five years, and even agree to spend no more than a certain amount - most likely, a fair amount of calls will be “past the checkout”. Yes, the announcement should be short enough. But not at the expense of content.
Evaluate the situation from the point of view of the reader (buyer, applicant, customer, etc.). Try to put yourself in his shoes. If you are looking to buy yourself a used sofa, what will interest you first? Dimensions, condition, upholstery color? Write about this, and not about the fact that "he served faithfully for three years and gave a lot of pleasant minutes." If you are posting an ad on the Internet and it is technically possible to attach a photo - do it.
Don't be silent about essential details. Do you want to hire a secretary, but offer a salary below market? So write. Yes, there will be fewer calls. But an effective ad isn't about the number of calls, it's about the result. And the one who only agrees to a high-paying job will not go to you in the end. But you will have to waste time on futile negotiations and raking three hundred resumes.
By the way, why is your proposal better than dozens of similar ones? Why, for example, you should be invited for wedding photography? Ah, are you a laureate of international competitions? Or vice versa - are you just starting to practice and are ready to work “for food” on a portfolio? So write. And be sure to select it (in font size, bold - whatever you like).
Don't forget to include your contact information. Moreover, indicate not only the phones, but also the permissible time for calls. People have very different ideas about what time it is decent, for example, to call a stranger on Sunday morning.