Watch television to see commercials advertising bargains and the like. Here are some occurences to keep your eyes peeled for:
A commercial for the Mauville Game Corner. When it plays, head over there for increased prizes.
A commercial for the Lilycove Department Store. Get a discount after you see the commercial.
Pokemon sighting information. When the TV tells you a specific Pokemon is spotted, go to that location to find the monster in question.
A commercial for the Slateport City energy guy. Go to him to get energy items at a reduced price.