nvest in a good quality trimmer. Any time you're sculpting facial hair, it's a good idea to invest in a beard trimmer to make the job easier. It's easier to shape your mustache accurately with an electric trimmer than with a basic razor you'd use to take all the hair off completely.
Usually, you can get a decent rechargeable beard trimmer for $15-20, available at most retail stores. They'll come with a variety of guard shapes and sizing options, making it easy to keep your mustache in shape.