Eliminate Noise on the Mix
As you go through your mix eliminate any noise or pops that may be in each track. Use fades as necessary to cut out any spots that may just be containing recorded noise. If this is done in the mix stage within each track it will keep the overall noise level down when the mastering engineer begins to equalize and compresses the mix.
Keep Your Mix Clean and Dynamic
Overusing processors especially dynamic processors (compressors) on the master bus can destroy a mix and make it difficult, if not impossible for the mastering engineer to make a great master. Unless there's a specific sound of a master bus processor desired for the mix, it's best to keep the master buss free of outboard processing or plugins. If master bus processing is used make sure to notify the mastering engineer of its type and settings.
The loudest part in a mix should peak at no more that -3db on the master bus.
There should never be a limiter set on the master bus. Final dynamic control and level should be left to the mastering engineer. This allows the engineer to create the proper dynamic level for radio play, CD, or mp3 duplication.
The two most popular file types for mastering are: .WAV and .AIFF. Both of these file types are lossless (non-compresses/converted), and either file type is excellent for a mastering engineer to work with. At DMR we work with WAV files.
When submitting a mix for mastering. The file should be kept in the same resolution as it was mixed in (no down conversions).
For Example: A song mixed at 24bit 96kHz should be submitted as a 24bit 96kHz file (.WAV)