When Submitting music for mastering please take the following things into account:

1) What is the Bitrate of your track? Is it in 16, 24 or 32 bit? Please if possible bounce your tracks in 24 bit or 32 bit floating point. 16 bit is Really not adequate, but if that is the what the track was written at it will have to do.
The Sample rate of the File should also be exactly the same Sample rate at which the track was written.
Is your track written at 192khz? Then that is the sample rate of the file you send for mastering. If the track was written in 44.1khz, please send it in 44.1khz. Up-sampling a track from 44.1khz to 48, 96 or 192khz will NOT make it better.

2) Are there any limiters/compressors/plug-ins on the master Fader?
Please remove all plug-ins from the master fader before rendering tracks for mastering.
Equalizers, Compressors, Limiters, etc... Everything!

3) Is your track Clipping at any point? / Is there enough headroom?
Before rendering your track in 24 bit, Please ensure that the loudest point of the track is at or below -3db. This space between -3db and 0db (clipping), is called Headroom and is ABSOLUTELY required before Fractal Sound can properly Master you music.
Be especially careful if you have written or mixed the track with compression or other processing on the master fader, and have since removed it.
If there is anything in your track over 0db, you get what is called Digital Clipping, and this is unacceptable. If a client presents a track for mastering with Digital Clipping present anywhere throughout the media, They will be requested to render another version.
It can be very useful to load your Track back into your sequencer and look at the waveform to determine how much headroom you have.

Take a look at the images on your right, to see examples of Good and Bad Headroom, as well an image of digital Clipping (zoomed in). This flattness in the waveform cannot be removed.