Good luck is all I can tell you! My mom never caved and let me get a snake. I had to wait until I was older, moved out, and married before I got one (course now I have to ask the wife!)
seriously, if you explain how they are not dangerous, they are easy to maintain, and promise not to use the snake to bother her (assuming she doesn't like snakes) or anyone else, usually this would work for me. Give them lots of information to show them that you have researched all about the animal and how to keep it healthy. This should at least show them that you are serious about it.