If you want a particular Pokémon, pay attention to the list near the lower right corner of the screen. It lets you know how close you are getting to the Pokémon that are in the surrounding area. Keep watching the indicator, and if you see the icon of the breed that you want moving toward the upper left position, that means you're drawing closer to your target. Fewer footprints also indicate the same thing. Of course, another option is to watch any other Trainers. If you see a bunch of people gathered around a particular spot, that's a clue that something worthwhile is in the vicinity.