Think of the mixing stage as merely creating a pleasing ‘mix’ of the elements of your music, with each element equalized to sound the way you want it, and as loud or soft in the mix as you want it. Then think of the mastering stage as bringing the mix to a commercial standard of loudness and making it sparkle — using processing — if that is necessary.
How to set up your mixes:
If you are using heavy compression/limiting on your Master bus, either remove that processor or re-set it so that you are getting only slight compression (1-2 db of compression). It’s better to have gentle compression throughout your mix (audio tracks and submixes) and leave 3-12db of headroom (whoa there! no clipping!) in your mixes. This will allow the odd transient to breathe, which is easy to control in the mastering stage.
If you’re recording at 24 bit (which you should be) there is no need for your mixes to peak at 0db Full Scale. You are already getting a wide dynamic range at 24 bit.
If you have a version you love with that compressor on the Master Bus, send it along (labelled) and I can use that as a reference.
Frequency-Wise: It is more difficult to master a song if the mix has uneven frequency distribution — ie: your acoustic guitar picking is on the dull side but your snare is very bright — which means that if I try to brighten up the frequencies on the acoustic guitar I will make the snare too bright. If possible, leave your mix sounding even, but a little on the dull side, and between eq-ing and dynamics, the mastering stage can bring out some of the life and clarity residing in the high end.
Leave a few seconds before the start of the song and after the end (of either the hard stop or fade-out). It allows the mastering engineer to hear the background ‘noise’ of the tracks, and gives some leeway in where to start and end the track itself.
If you have alternate versions (edits, narrative version without lead instrument), make sure the mix is the same as the main version — full mastering of alternates will be charged the same rate as the main mix.