How do I get my parents to say yes?

When I was about 9 or 10 I had a hamster, and wasn't too responsible about it. We have a dog, a cat, and a fish... but none are mine. The dog is a mamma's boy, our cat just lays around all day (and is very fat) and a fish is a fish (can't do nothin about it). I really want a pug or even a turtle, but I cant convince my parents that I'm more responsible. I'm 14 now, and I already know that the problem isn't that we already have enough pets... because we actually used to have like 5 more. Anyone know any tricks or strategies to get my parents to let me get a pet (maybe even for my birthday)