One of the things I do that helped a lot with the quality of custom ringtones on my previous phone (Tilt) and on my Expo now is to use a program like WavePad or Audacity (they're free) on the PC to trim the sound file to exactly the length I want, the part I want, and sometime to even loop it, if it's a short clip.
More importantly though, is the fact that CD-quality MP3 clips often end up sounding like a distorted miniature kazoo band when they're squeezed out of a teeny-tiny phone speaker like ours. So for most ringtones I generally do the following at least:
1) add in a high-pass filter to take out the very high end of the frequency range
2) cut out the frequency range below 100hz completely
3) add a light envelope filter
4) add about 20-25% compression overall to keep it from sounding "mushy" or "underwater"
5) save the altered clip at 192kpbs quality to save space (above that, it doesn't sound any better anyway through our speaker)
Hope that helps, or at least gives you ideas!
OH, and welcome to the forum!
