The most important thing to keep in mind is to avoid clipping. Make sure your master channel is never hitting 0 dB, and ideally aim to have the master channel hitting between -3 and -6 dB.
Try and export the audio at the highest possible format both in terms of bit depth and sample rate (a good number to aim for is 48 kHz sample rate and 32 bit depth) which will provide eMastered with the highest quality audio from which to work with.
Additionally do your best to remove unnecessary frequencies from the individual tracks in a session before bouncing your unmastered file, as removing these frequencies will provide a cleaner product for eMastered to work with and will provide better results.
Finally if possible remove any plugins you can from your master channel. These plugins can negatively impact how eMastered will improve your song, so to get the best result we recommend removing plugins on the master bus before sending to eMastered.