You have two major options:
- Right click and choose get info (or apple-I or whatever) and then one of the advanced tabs has start / stop point options. You don't get any fades or niceties with this, aside from the crossfades in iTunes.
- If it's not DRM'd music, you can certainly also load it up in Sound Studio, GarageBand, Audacity, etc, and edit it there, and then reload the modified version back into iTunes. This would let you add a fade into it, etc. You can also make ringtones from DRM-free music this way.