gswitz
I usually mix so the loudest peak is to -0.1.

According to several posts on the forum that's too loud for MP3s. The conversion makes the MP3 clip
if the original wav is that loud. The recommended level seems to be about -0.3. I have no experience as I've never made an MP3 version. Just repeating what I've read here.

I agree regards the -0.3 threshold, it just lessens the risk of it being replayed on an out of calibration environment.
I have just got the dp meter mentioned in the deals forum which is worthwhile. Can't add it to the master so create a pre-master bus and run everything through that first (with nothing other than a limiter and the meter*) either direct from all the tracks or through staging and effects busses before the pre-master bus.
*quad band eq and quad band compression before the limiter if you absolutely need it 

In theory the technique for exporting MP3 is exactly the same as as exporting WAV. I'm not aware levels need to be different from wav, or anything else needs to be watched out for especially... News to me anyway..?

I think if you want to optimize for MP3, make sure your music has more samey and simpler waveforms. Make it repeatative. Make it house music. So anybody who exports classic or dynamic rock music to wav, should probably do housy housy version especially for mp3 exports...

The other big thing to take into account: you may have a good deal of volume in the areas that can't be heard, very low frequencies and high frequencies.  If you remove those, then the rest of the mix can be shifted up a good deal.

Doktor Avalanche
In theory the technique for exporting MP3 is exactly the same as as exporting WAV. I'm not aware levels need to be different from wav, or anything else needs to be watched out for especially... News to me anyway..?

There is actually quite a bit of informed commentary available that the process of transcoding from wave to MP3 can introduce clipping if the wave gets too close to 0db. Most recently reiterated by Mr Anderton here:
 
http://forum.cakewalk.com/FindPost/3294962