It is important that when you need someone to work for your company that the ad you place is one that is will bring in only the candidates that are truly capable of doing the work that you need to have done. With all of the job seekers there are right now on the job market, this becomes even more important. As an employer you don't have time to go through a ton of resumes that simply don't have the skills that you are working for.
The most important thing you need to do is truly understand all of the requirements for the job that you need to hire someone for. Even if you are the boss there is a possibility that you really don't know all of the requirements that it might take for every single job that is done in your office. So it's a good idea that you talk to others that may do similar work or have done the job previously before they moved onto a different role in your office. You will also need to make sure that you use key words that will help describe the position that you need filled. For example if you are hiring a receptionist make sure to include things like “professional appearance”, “loves working with the public”, “needs to be outgoing”, etc. You will also need to include how many years experienced that is required, include whether it's entry level or not, include educational requirements if any and so on.
Before you place the ad it's important to know what the budget is for advertising with your company. It costs money to place most employment ads so know ahead of time what you can spend before you make up the ad. Once you know how long it can be, then you can sit down and see how much information you can include. The shorter it is the less information you can add so you will need to decide what factors you want to include that are the most important to you.
Your best place, although it's the most expensive, is to place your ad in the newspaper in the classified section for employment. You will find different categories and if you can afford it you should consider placing the ad in different categories that apply to the kind of work it is. The more exposure your ad has the better response you are going to get.
If you are looking for an entry level person you might be able to stay away from the newspaper and place the ads local college papers, on Craigslist and other online job search sites. However if it's a high level position it might be best if you enlist the help of a job recruiter also known as headhunters.
Make sure that you try to add some of the perks and requirements of the job to the ad if you can. This could be a warning that the job requires travel, or the job can be done at home, paid benefits like health or bonuses, whatever you think might attract the right candidate.