To unlock the Hello Cleveland! trophy in Rock Band 2 you must use Vocal Overdrive four times in one song. The best way to do this is to pick a reasonably long song and play on Easy. Obviously picking a song you feel comfortable with will give you more opportunities to earn Overdrive.
Good song choices include:
Nine in the Afternoon
Let There Be Rock
Testify (if you're tone-deaf - that one's mostly mono-tone rapping)
Hungry Like the Wolf
If you hit Vocal Overdrive four times in a single song play through, you'll unlock the Hello Cleveland! trophy.