I never get tired of saying that behind of a great master, there is always a great mix, love your mixes and work hard on them, the final result will always be superior. If you already have a good or great sounding mix, here you can find some basic tips in order to prepare the files before you send them at the studio.
I accept a large audio formats, but I prefer WAV or AIFF stereo interleaved files at the same sample rate you recorded the song (or the same one used during the mix). I also strongly recommend working at 24 bit or higher resolution, you get 6dB of headroom for every bit added, resulting in 48 dbs more working at 24 bit than making the same at 16 bit, although the final destination is an Audio CD, you'll benefit from this extra dynamic range during the Recording, Mixing and Mastering stages, 32 bit float point are welcome too but 24 bit bounce will do the job.
Make sure to leave some headroom for the engineer can do his job preventing unwanted distortion, take care that your highest peaks are between -6dBfs and -3dBfs, peaks may to exceed the -3dBfs occasionally if no distortion, but if constant, please, put down the individual tracks faders in the mix instead the master bus fader.
MASTER CHANNEL PROCESSORS
Try to remove any "finalizer" type plugin from the master bus (i.e Limiters, Compressors, Multibands, Maximizers, etc...), unless you use them for artistic purposes, gluing, color. etc... if that is not the case, please, remove them, if you need volume I have got special hi-end tools for that job without compromising the whole track dynamic. If you still believe that they are good for your song, then send me the compressed (as reference) and uncompressed version to work with.
Be careful with the noise on the mix, things like vocal sibilances, clicks, pops or background noise will be further enhanced after the mastering, I can remove some noise, but the final result is significantly better if these problems are corrected in the mix. The same thing for psychoacoustic processing such as stereo widening processors, abuse of them may break the balance of the mix and creating phase shifting problems. Leaving a small space at the beginning and end of the song is always a great idea, this way you will avoid mistakes such as cutting the first transients on the track or the reverb/effects tails at the end.