I used to turn around and not look at the screen, and it makes me think that I have a way better chance. Because I don't know if its a coincidence or not, when I tried like 15 times to catch a stupid gligar w/ an Ultra Ball, it didn't work, then I used another one and turned around, and I caught it! Same with the legendaries, when I look, I fail, when I turn around, I succeed!