If you decide that your use is probably not a fair use, your next step is to ask for permission.
Identify the copyright holder
In the case of print material, the author is usually the copyright holder. However, the pubisher may hold the rights. In order to determine this, look for a copyright notice on the title page. Unfortunately, not all works will include a copyright notice, and it is also possible that the copyright has changed hands since the notice was printed. The U.S. Copyright Office has a catalog of works copyrighted after 1978, as well as a publication on how to use their resources to determine the copyright status of a work.
For older works, especially for materials like photographs and audio recordings, it may be impossible to identify and locate the copyright holder. Always keep documentation of your search for a copyright holder. There is risk associated with using these works, and in the event that you cannot find the copyright holder it may be best not to use it. If you decide to use the material anyway, documentation of your search could prove useful in the event of legal action.
Ask for permission to use the work
Many publishers prefer that you make your request using a form on their websites. Others may require that you make your request via fax or email. Whenever possible, make your request in the format preferred by the copyright holder.
If the copyright holder does not have a set form for permission requests, send a letter. Always keep copies of your correspondence, especially the signed permission forms. If you are sending your letter by mail, include an extra copy for the rights holder to keep, and a self-addressed stamped envelope for the reply.