I've just discovered that a good way to get more pokemon is to kill the app and then restart it. Each time you do it rerolls the available pokemon in your area. I don't know if there's a limited number of pools or something, but I went out on some errands today and using this trick I managed to get five pokemon, including a fully evolved Pidgeot in my backyard when I got home. I also got a Paras, a Zubat, and a Spearow.
And of course there was the additional junk Rattata.