Providing an accurate description of the music you need can help a lot pitching the right song. A good brief will encourage your contacts to submit only the tracks that respond to your requests, and not all kind of stuff they have in catalogue, from black metal to country music.
Music providers might need:
Music genre: sometimes genres overlap and the differences between them are not so neat, but this is not a good reason for an unprecise use of the definitions. Then, be sure that the word you are using fits the type of song that you are looking for. Also, don’t invent definitions. If the name of the genres don’t fit the idea you have in mind, don’t mix them, like “afro-funk-beat”, just add more information regarding mood and instruments.
Instruments: instruments can help if you don’t have an exact idea of the music genre. To tell if you want a vocal or instrumental track it is also important.
Mood: provide the sensation you would like the music to evoke. Focus on the track, not on the scene. For example, a lullaby in a horror movie can cause a creepy feeling, but to get what you want you would have to write “chilldish mood” better than “creepy mood”.
Tempo: you can give information regarding the tempo both by writing descriptions (slow, fast, mid-tempo etc) or providing the bpm.
Style: describe what kind of scene you are working on. A love scene, a chase, a ball, an epic battle. This will help your providers to imagine the music in association with the images.
Synopsis: you can also help providing a short abstract of the scene, indicating how the song will be used (background music, sung by actors etc.).
Sound like: it is very useful to provide an example, a song that you heard and that could work very well with the scene.
Deadline: often it happens to have a last minute request. It is important to specify how much time people have to submit music, in order to collect all material before the day the production need it.
Budget: tell how much money you can spend for the track, so that your providers are already prepared to exclude too expensive ones. Otherwise you might risk to find the perfect song and then discover that you can’t afford it.