I’ll share this one:
When you prepare a (pre) master or final mix for mp3 encoding, it’s best to peak at (not higher than) –0.3 dB. Although most DA converters nowadays can handle 0 dB or –0.1 dB, mp3-encoding isn’t that allowing and distortion will appear fast. I didn’t know this until recently and the information came just a tiny bit too late for me; one of my mp3 songs is clipping (first song on my myspace at the moment of posting) where the original AIFF was/is all good.

Peace,
Merlin.