The recording and mixing is finally completed and you're really happy with the final mixes. Now, you're ready to get your tracks mastered. There are a few things you need to make sure are in place before you send your tracks over to the mastering house.
First, make sure your peaks don't exceed -6dBFS. There needs to be some headroom for the mastering engineer to work. When you send your mix with no headroom, the mastering engineer is limited by what they can do. When I've worked on tracks that have no headroom, I have to spend time with the gain structure to allow for any adjustments that will enhance the mix.
Second, send uncompressed .wav files for mastering. It is never a good idea to send in .mp3 files for mastering. The lossy codec of the mp3 has stripped away rich audio details that the mastering engineer will need to deliver a quality master. It's best to send files from their original sample rate and bit depth. If you tracked at 96KHz, then send the mastering engineer 96K .wav files to master. The mastering engineer will do the final dithering and make sure that all files are formatted for public distribution. If you're unsure of what to send a mastering engineer, just ask them. A mastering engineer will be more than willing to provide you with details about what to deliver. This will save a lot of time if there are any problems with the files you send over.