another thing to add, well a couple,
if you're using mp3 a lot then get to know how frequencies are handled as well as, finding out the frequency that drops like a rock (i think it's 16Hz or higher but i can't remember right now) this isn't easily compensated for and unlike the transition from mixing for vinyl vs cd, most pro's haven't dedicated their time in figuring out or even trying to mix tracks with mp3 in mind (not only in terms of reference tracks converted by them but also the bought and sold retailed cd that gets converted to mp3 and aac, usually at 128-192 in order for people to put more songs on their player and cd)
mp3 and conversions need more headroom, if you have your final at something like -.3 or slightly lower you need to lower it even more. unfortunately, there's nothing available that can can preview or relay info prior to conversion unlike something like an intersample distortion meter to avoid intersample peaks but conversion is quick so it's trial and error.
another would be converting directly from 24 bit if your working in 24 bit (or higher such as 32 bit float) , and doing it without dithering