how do I normalize(volume) over my music ibrary in itunes?????

This is Driving me nuts, I have ripped my cd library to Itunes, and have purchased 100's of dallors of music from Itunes and nothing is normalized! I also have Icloud which automactically downloads these songs from my main mac to my other 4 ipods and 3 macs, and when I want to rock, I spend all my time turning the volume up> or <down for every song. On my Ipods I have the sound check on, but it does not seem to do anthing. Even the music purchased from Itunes has a big discrepencie in volume levels! help! help!