How can I prepare my mix to get the best results from
mastering?

LANDR is designed to be nearly foolproof, but, like any mastering
service, or relationship, benefits from a little bit of breathing room.

First of all, make sure the master output isn't clipping. In fact,
you should leave several dbs of headroom (keep the bouncing
master level meters well below the red, ideally at -4 or -5dBs).

Secondly, avoid compression, limiting or EQ on the master
output of your final mix. Only apply these processes if you
feel they are musically essential, and try a version where they are
left out completely.

Finally, export your mix at the highest possible bit depth (ex: 24bit)
and sample rate (ex: 48kHz). This shouldn’t make a huge difference
over 16bit / 44.1kHz, but the more resolution LANDR has to
read from, the more precisely it will make its adjustments.