The Magic of Stereo
Reducing bit and sample rate flattens the depth of a recording. Adding a stereo source (not just multi-mono) increases depth and dimensionality, allowing those characteristics to hold up as the quality is reduced. Drums are a great candidate for a stereo foundation. And make sure to check the mix in mono to verify you're not getting phase cancellation.
Get it Right at the Source
Although equalization is a fantastically powerful tool, even good equalizers can make a mix edgy when accumulated over dozens of tracks in a mix. Spending the time to get the sound you want at the source will result in a fuller mix that holds up as the bit and sample rate are reduced.
Know Your Audience
Earbud headphones, car stereos and home theater systems usually have a lot of bass and high end, but don't reproduce midrange as well. Make sure you don't short-change the midrange in your mixes. A good mix — with the right balance of low, mid and high frequencies increases the clarity, impact, power and detail of your music... and this will survive all resolution formats.
We're in a world where we have more options at our disposal than ever before, but we also have threats to sonic integrity that none of us can control. Your recordings can sound good in any format, but this requires you to take the proper steps in ensuring they have a fighting chance. Do your part to make sure your mixes sound their best... no matter what happens to them after they leave your studio.
Paul Abbott is a professional mastering engineer and owner of ZenMastering, based in San Diego, California. www.zenmastering.com