How do I equalise song volume on iTunes/iPod?

I know about the slider in 'get info'. Maybe I'm doing it wrong, but all this does to the songs on my iPod is make them reach their maximum volume quicker (i.e. they reach their maximum volume when the volume bar is 75% full then don't get any higher). I want the actual maximum volume to be higher on those few pesky songs than seem to be quieter than all the rest. Help?!