Preparation for Mastering
The preparation for mastering is very important because it can mean the difference as to whether or not a song can be mastered or cannot be mastered for various reasons.
For starters you always want to have a good mixdown of your song. You want to have a good listening environment where you can adjust to hear all the tracks in the recording so that everything sound wise can be heard during playback.
All the sounds should also be panned in the stereo field to give a more spread out sound that wont sound like everything is on top of each other.
There should be a minimal use of processing on the final mixdown without too much bass or treble that would muddy or thin out the sound.
One of the most important things is the level of the mix. It needs to not be anywhere near 0db or exceeding to the point of peaking. The mastering process is going to gradually cause the audio to become louder so if the song is already loud in the mix, applying the mastering process may distort the sound.
If these things are discovered by the mastering engineer, the project most likely will be rejected. Any professional mastering engineer can listen or look at a waveform or overall levels to tell that a song cannot be mastered. There just wont be enough headroom to work with.
So if you want to be sure your music is prime an ready for mastering it is crucial you adhere to guidelines for getting your properly prepared.