Don't turn that dial! (Ad agencies are going to love me for this one!) :) The majority of commercial voice over work is in the form of radio commercials. When your favorite song stops playing and the DJ starts blathering before cutting to a commercial break, don't immediately turn the station. Listen to the commercials, even the bad ones. This is your competition and a large part of the work that you'll be auditioning for. Take notice of what commercials catch your attention, or ones that you think you can realistically be doing someday yourself.
If possible—and without causing an accident—you may want to try recording a few.