Once your track is finished, how you export or mixdown your track is very important when it comes to mastering.
Remove any compressors or limiters from the Stereo / Master output channel. For good mix practice there should not be any plug-ins on this channel.
Turn the master / stereo output fader down so the peak music level does not go above -4.0dB. There should be some space between the peak music level and the top of the level meter.
Mixdown your track in the normal way in the format WAV 24 Bit 44.1 KHz
To check your WAV is mixdown correctly open it with your normal programme or Audacity (download free here
http://audacity.sourceforge.net/). It should look similar to this waveform