How do I get my song ready for the Mastering Engineer?

Taken from:
http://www.x2mastering.com/faq_ready_for_mastering.php

Through no fault of their own, musicians and mixing engineers often misunderstand exactly what a mastering engineer wants. Some mastering engineers like to pretend that there is some hidden aura around what they do, and that they can work magic with whatever you give them. The truth is that these mastering engineers only make their own job harder. There are a few simple guidelines to follow when preparing your mix for mastering:

DON'T MAKE IT LOUD!

If you only read one thing on this page, please make it this paragraph. It's perfectly fine if your mix doesn't sound nearly as loud as your favorite album — it's not supposed to yet. Part of the process of getting a song mastered is increasing its loudness. Of particular note: If you record and mix 24-bit audio, your mix could peak as low as -20 dBFS without any difference in the loudness or quality of the final CD master. You should feel comfortable sending mixes that peak around -6 dBFS to your mastering engineer. Most of the energy of your mix (RMS) should be around -18 to -20 dBFS. It doesn't matter what genre you produce when it comes to mix loudness - let the volume come during mastering.

What Format?

High-resolution data files are preferred, especially for clients who choose mastering online via FTP. Save your songs as 24-bit WAV files if at all possible, but 16-bit files are also acceptable. Sample rate matters less, but here they are from most to least preferred: 44.1kHz, 88.2kHz, 96kHz, 48kHz, 192kHz. If you already work in one of those sample rates, please don't convert it to another before sending it to me. Also, if you mail a CD, please make sure it's a data CD and not an audio CD.

Wait, I thought the highest sample rates were the best?

While higher bitrates are always preferred, higher sample rates aren't always required. When recording, higher sample rates will be more accurate and true to the audio being recorded. However, mastering engineers typically prefer to apply as few processes as possible to projects in order to maintain fidelity. Since a CD is burned at 44.1kHz, I prefer to work with files that are 24-bits and 44.1kHz. Reducing the bitrate is always part of the last step in CD mastering, because the rest of the process is typically done at 32, 48, or even 64 bits.

Edits, Fade-Ins, and Fade-Outs

Fading tracks in and out is not commonly understood to be the mastering engineer's job among indie artists. It's fine if you have a few seconds of "dead air" before and after your song. In fact, it makes some tasks like noise/hum removal much easier. If you trim a song so that it begins with the music and fades out to digital silence, it can be much harder to isolate a sample of pure noise or hum for the cancellation and removal process. Fade-ins and fade-outs can be done with extreme precision during the audio mastering process; simply let me know where you want the tracks to begin and end, and how long you want any fades to be.

Stereo Mix Bus Processing

It is typically best to leave any stereo mix processing to the mastering studio. If you have an equalizer or compressor across the master bus, or affecting the entire mix, please disable it when you prepare your files for me. By all means, print a second copy with the bus effects activated so I can hear the direction you want your mix to go. Very mild processing, such as 1 or 2 dB of compression with your favorite sounding compressor, is perfectly fine. This tip more applies to those who try to get their mix to sound "huge" or "loud" or before it's supposed to sound that way.