I'd like to offer some advice about mastering. Many tracks come in to the studio for mastering and sometimes the mixes are not ready to be mastered. So here are a few tips on how to get your mix ready for mastering.
The primary thing to keep in mind when mixing is to make sure that you leave enough headroom in the mix for a mastering engineer to work their magic. So many times I get a mix that has absolutely no headroom. The dynamics of been squished to a point of no return and it is virtually impossible to master these tracks. Be conservative with the volumes of your mix. If you want your music to be louder just turn up the volume on your monitors. Do not use a limiter or a compressor to squish the music and boost the level. It's a good idea to not sacrifice the dynamics of your mix for increased volume levels. Use meter software and make sure that your mix has at least 6dB of headroom. This means that the maximum peak level of your mix does not exceed -6dBFS.
Dither is another area that can be confusing. To keep things simple, don't use dither if you have no idea what it does. If you track at 48KHz | 32 bit when recording and mixing, then make your mix at the same settings. The mastering engineer will do the dithering for you and make sure that the audio fidelity is maximized. If you convert your files to a different sample rate w/o dither, then you could induce artifacts or digital distortion into your mix.
If you're not quite sure how to deliver your files for mastering, then just ask your mastering engineer. Be sure to send the best quality files, too. Don't send MP3 versions if you have WAV files available. There are many sites that allow you to transfer big files for free. Sites like wetransfer.com is a great site for file sharing.
Hope this gives you a little more insight. Please leave comments and questions.