Plug-ins on the Master Output
If you are using any limiting or compressors on the master output during mixing, then export two versions: A) one version with plug-ins enabled on the master output, and B) one version with plug-ins bypassed on the master output. Limiting or clipping should be avoided on mixdowns for mastering.

Headroom
Your 24 bit mix should have its highest peak between -12 dBFS and -3 dBFS. This translates to between 12 and 3 dB of headroom before exceeding the digital ceiling of 0 dBFS.

Headroom is the amount of dB before your mix clips and the overload indicator lights up on your master output. If your mix is too loud then simply lower the output fader until the highest peak is within the recommended range. Sound quality will not be affected when you lower the fader. With 24 bits you can go as low as -48 dBFS and still have full CD quality (16 bit). However, once you exceed the digital ceiling, distortion will occur, and it will not be possible to restore the original quality. There is no reason to maximize the volume during mixdown. I will make sure your song reaches its maximum loudness potential later in the mastering process.

Start/End
Export your mix at least one bar before it actually starts. Include a couple of extra bars at the end to ensure reverbs, delays, and instrument decays have tapered off completely.

Audio format
WAV or AIFF is preferred. FLAC is also fine.

Stereo Format 
Choose interleaved stereo

Bit Resolution
24 bit is preferred. 16 bit files are only used whenever it is not possible to get a 24 bit file of the same mix.
32 bit floating point is not used for mixdowns for mastering. The actual sound contents of a 32 bit float file is identical to that of the 24 bit fixed format, it takes up more space, and is slower to data transfer.

Sample Rate
44.1 kHz is completely fine. Only export in higher sample rates than 44.1 kHz if your project is actually recorded and processed at that rate. If your project contains different sample rates then do not convert sample rates yourself, let the me do it.

Dithering and Noise Shaping
Do not use noise shaping or colored dither, e.g. UV22 or POW-r when exporting your mixdown for mastering.

Normalizing
Please do not use any kind of normalizing on your mixdown.
Normalizing raises the signal level in an unnecessary fashion, and it will change the amount of headroom left in the mix. The final volume level of the mix will be optimized by me.

Listen to Your Mixdown
Always listen to the exported file from the beginning to the end before sending. Make sure everything is playing correctly, that the beginning and end are intact, and that no artifacts or click sounds occur.

Sending Files Over the Internet
Before sending a file over the Internet you should pack it as a ZIP or RAR file to maintain data integrity. You can upload the file to any data cloud service such as Dropbox or Google Drive or you can also use wetransfer.com and send it via email to dsphsounds@gmail.com