People are generally so timid about asking for anything that people actually don’t have much of a defense for rational requests. At most you might have to back up your request with a few logical statements.
The first step to asking for something is how you ask. Watch your posture and self-control. Be assertive but try not to demand something or be aggressive. You need to go into the conversation prepared and confident but not like a bull looking to lock horns.
Beyond being able to show measurable proof of your success and your worth, put together a few ideas on how you can be even more valuable. You may be tapped out on the salary for your current position but what if you added a few responsibilities.