Tips at

Consider removing bus compression, EQ and other processing applied to your Whole mix

Having the unprocessed (no bus processing) helps with mix analysis and mix diagnosis. If you have used bus processing on your mix, and you are comfortable with it, there is nothing wrong with leaving it on but consider sending both a "processed" (with bus processing) version of your mix along with a "raw" version. By providing the two versions, Rob can provide you with more mastering options later. You can read more detail about that here:

Watch the peaks.

What that there are no 0dBFS peaks in your finished mix file. If you set the gain on your mix so that the highest peak no higher than
-3dBFS, Rob will bring the loudness back up as necessary, during the mastering process. 

Send mixes without the fade outs (if applicable).

Any dynamics processing applied during mastering will have an audible impact on the fades. Depending on how much processing is applied, the fade outs may appear to drop suddenly compared to a nice smooth transition. For this reason, it is best to finish the mastering of each mix, and then apply fades after that process has completed.

Send high bit-rate, high resolution, uncompressed files.

24-bit 96kHz *.WAV format is recommended (lossless formats such as FLAC are also fine). If you did not mix your project at this rate, please send your mixes in the same bit depth and frequency that you used (i.e. avoid upsampling later). Avoid sending "lossy" formats such as MP3 or AAC that remove pieces of the audio. Working directly from a raw, uncompressed source file will give the best results. 

Tips for great sounding mixes

Check your mixes for mono compatibility.

Even though we live in a Stereo and Surround Sound world, Monaural (a.k.a. "Mono") is still alive and well. In fact it is common for DJs and live engineers to work in Mono. Stereo is less important in a live venue due to the acoustics and the fact that the audience is often not staying in one listening position all the time. When you switch your mix to Mono, does the sound quality change dramatically (bass disappears or vocals are harder to hear)? If so, review each of the tracks in your mix to find out what the source of the issue is (mic bleed, inverted polarity etc).

Get Extra Creative With your Reverb

A lot of reverb effects units and plugins enhance the bass in the reverb to make it sound full. Some have an enhanced top end, too. If you use reverb on several tracks, it can build up and create "muddiness" in your mix from the excess bass. It can also cause  harshness in the top end depending upon how the reverb program responds to the program material. You can avoid this by equalizing what you send to, or get back from your reverb. Solo the return so that all you're hearing is the reverb, and then use the EQ to remove portions of the reverb that are not important to your mix. You can find more detail here:

Know your gear - including your software

There is a lot of great gear available today, including software plugins, and many of these products do more than just one thing at a time. Get to know exactly what your gear is doing to the audio - especially if using presets. Using the "Hit Mix" preset on a buss compressor may sound great at first, but it is important to understand exactly what it is doing. Is it squashing your dynamics? Is it adding unwanted noise or harmonics to the signal? Be cautious of plugins or other software that have really slick interfaces and graphics. Most can be great tools if used correctly, but it is easy to trick yourself into thinking you are hearing quality audio because your brain likes what you see on the screen. Close your eyes, listen critically and let your ears be your guide.

More mix tips, information and resources 

Tips for recording engineers

Watch your levels.

Audible distortion in a recorded track is next to impossible to remove. At best, you can mask it a little, which is a compromise to an otherwise great sounding mix. Being conservative with recording levels.

Bit depth matters.

24-bit is significantly better than 16, and it is well worth the extra storage space and processing time required. If you regularly work in 16-bit, 24-bit audio will make a real difference to the quality of your productions. High quality 24-bit audio offers a noticeable reduction in noise over standard (e.g. triangular) dithered 16-bit, especially when used within a mix made up of multiple tracks. You can also use lower recording levels when tracking at 24-bit which will further help to enhance the quality of your recordings and mixes by minimizing the chance of distortion.

For the highest fidelity and flexibility, use the cleanest and shortest recording path possible.

Use high quality microphones, with the shortest possible cable lengths with good quality cable, and clean connections running into an uncolored preamp will ensure that you have the very best starting point to work from when you begin mixing. There are many products out there that advertise having "character" or "warmth". Be very careful - sometimes it is nice to have in certain situations but if that is the only preamp you have, everything you record will be stamped with that character, which will soon become tiring. You can always add character later in the mix if you start with clean, uncolored signal.