Mix Level: I second 85db. It's pretty much the defacto standard.
For
MP3 encoding that's one of the biggest issues with any of the lossy formats, they all give somehwere .. but also good to know is that not all encoders and/or presets are equal. LAME has a ton of options, and I find that some of their presets (like studio) are a pretty good tradeoff for encoding time / filesize / sound quality. If you want high fideilty you really need to use a non-lossy format.
Panning: There's another trick that someone taught me a while back to check the spacial properties of your mix. Always check it in mono as suggested, but another neat thing to do is to flip one side of the stereo pair's phase and then check it in mono again. Try this on some commercial music mixes as well. It opened my eyes to the uses of phasing as well as panning as a tool quite a bit
Be careful though, this can be a very slippery slope .. too much in the way of "phasing tricks" can be bad, while a little one here and there can really bring things out. This is paricularly important when you're using any sort of mid/side micing techniques as well. Being subtle is the key here.
Surround: I've done a bit of surround but not a lot (and not on my rig, I'm not setup for it -- as nick said -- yet hehe). If you're not going out to a surround medium for the final mix I don't think this would gain you much of anything in the end except headache.
The Master: As nick said about 24bit outputs for masters (you should ALWAYS USE THESE IF YOU CAN) ... but it is potentially different than you would approach with a 16bit outputs. There's not an easy explanation for why, you have to read up on the differences between 16 and 24bit audio and that can get pretty convoluted depending on your sources! Moral of the story, use 24 bits to output for master if at all possible.
Just my $0.02
Mark
