Don’t use effects on the master output/bus/track – compressor, EQs, limiters etc. This can really limit what the mastering engineer can do for your music. Instead, allow the mastering engineer some headroom and they will be able to bring your track to life, as the music will respond better to their mastering efforts.
Listen to your mix on as many different sets of speakers as you can – large and small – to make sure you’re happy with how it sounds in different environments. Try your main stereo, the car, an iPod dock and so on. Good audio recordings generally sound good everywhere, and the better your music sounds before mastering, the better it will sound after mastering.
Ensure that none of your audio tracks are clipping, either on the track itself or within a plugin.
Ensure that the master isn’t clipping, but peaks at as high a volume as possible.
Bounce your final mix in as high a resolution as possible. The higher the quality, the better results you will get from your master. If you recorded audio at a sampling rate of 48k and a bit depth of 24 then make sure you bounce at 48k and 24 bit to ensure quality is not compromised at any point.
Try to avoid converting your bounced file to MP3 prior to mastering. MP3 audio quality is much lower than that of a WAV, AIFF or FLAC file. Save MP3 conversion until after the mastering process.