How would I go around correcting a lisp?

If you're aware of the problem you may just be able to recruit some friends or family to help you practice. Have them explain in detail how to make the correct sound and then you try it. Once they acknowledge that you've done it correctly concentrate on how it feels to say the words as they should be said. Eventually it will become natural and when you slip and revert back to your old way it will feel wrong and you'll be able to catch yourself.