In my experience as a mother, there is no surefire way to make your mom say yes. You can try giving her a well-thought-out and intelligent argument and see how that works. You might write down the reasons why you think she should say yes and try to reason with her.
We think your Mom will say yes if you ask her if she loves you. If you are trying to get her to agree to something try to explain why it is a good idea or make promises to do things that will make her happy and then keep your promise.