There is no nice way to normalize an mp3 without re-encoding it entirely which degrades quality alot and it is not reccomended.

Here is a nice program for normalization/mp3 encoding
http://www.poikosoft.com/cdda/
It's a cd ripper, but you can just ignore that part of it. It also comes with a nice little graphical front end for encoding mp3's and includes the LAME mp3 encoder. (one of the best around)
It should take mp3's and wav files as source for encoding, but if at all possible get a clean wav file to encode from rather then re-encoding.