1) Put all necessary details in description but keep it as short as possible. Use short description with phone number, if applicable. But don’t use your personal email address. Use anonymous email feature if you need to receive email queries from the visitors. 

2) Use image whenever possible. In the categories where you can’t use external image and need to upload the images, image size is restricted to 300×300 pixels. If you upload images more than this size, craigslist will downsize. So it’s better to use image not larger than 300×300 pixels.
3) Use attractive titles which clearly describe what you are offering. Avoid using excessive symbols and capitals. Lenght of title is limited to 70 characters only. If you title is more than 70 characters, craigslist will discard the extra characters which may make your title meaningless. Be careful not to use titles longer than 70 characters.
4) Use your webpage URL in description, if you need traffic to your webpage. Use multiple domains if required. Free URL forwarding services like tinyURL.com will not work. So you must register your own domain to redirect visitors to your webpage.
5) Post the ads on appropriate time. Proper posting time varies depending on service or product.
6) Post the ads under the appropriate category and appropriate locations. Posting ads under wrong category or location may compel the visitors to flag your ads.