I have mentioned in the past - see our post Recording Engineer, Mix Engineer and Mastering Engineer – Oh My - that there are really 3 distinct jobs when recording, especially for music:
When we record from our home studios, we often do all three things, but there is a lot of benefit to drawing at least a mental line between all 3 jobs. It helps us to refocus and think a bit differently about what we're doing.
But if you are going to send your music project to a mastering engineer - which is a really good idea, BTW - then there are some things you can do not only to make life easier for the mastering folks, but to get your project produced faster and sounding better.
In this article, Bobby Owsisnksi outlines 8 tips for making your mixes more master-friendly. I would add one tip to the list though, and that would be to leave some headroom in the final mix-down (rendered stereo file of the mix). This gives the mastering engineer room to work with and optimize the final level of the song. If you send in a compressed and normalized audio file, the mastering engineer doesn't really have breathing room to allow space and "air" to come through in the final version.
But without further ado, here is the original article to which I refer: