Mixing Tips
Here are a few basic pointers to insure the mix is suitable for mastering:

Don't use any form of compression or limiting or clipping on your output bus: No saturation, inflator, tape simulation, valve drive, etc. This is best left to the mastering stage. All this kind of processing is OK on the sub busses and channels.

Don't allow any channels, busses or the output bus to peak over zero. When mixing "in the box" be very careful with plug-ins. Your channel or bus may be peaking below zero, but if you have inserted a plug-in, you may be clipping the signal within the plug-in. Some plug-ins have very little or no headroom, at least less than your workstation. All it takes is a bit of clipping and overloading here and there throughout your set-up and the mix starts losing clarity and sounding grainy and muffled. No tricks in mastering can fix this. Of course there are times when you want an instrument or voice to sound distorted in some way and perhaps overloading a plug-in works for you.

Make sure the bass drum and bass work together as a unit. If the bass is too soft in your mix, boosting the low end in mastering will bring the bass drum up as well. The same principle applies in other frequency ranges: If the lead vocal is too soft in the mix, boosting around the vocal range in mastering will bring up other instruments in that range which are already fighting with the vocal.

Make sure there is no "mud" below the vocals and guitars and all instruments not in the bass
range. This will reduce definition in the bass end of the mix. Removing this mud in mastering will make the bass end too light.

If your track is not working when listening back to the un-mastered mix, strip the mix down and start again. No amount of mastering can fix a bad mix.

The way a song sounds depends firstly on the standard of the musicians, the quality of their instruments and voices, the arrangement and production, and then thirdly on the way the song is recorded and mixed. Mastering is the fourth and final stage. Each of these four stages is of equal importance. If there is a weakness in any of these four basic stages the production will suffer, and the weakness cannot be rectified by a later stage in the production. Some mastering studios claim they can "fix" a problem like this in mastering. This is a fallacy. Get the first three stages right!