Have you actually done anything promotion-worthy?
- Stayed later than necessary to get that project finished by the deadline?
- Taken extra time to mentor/assist more junior colleagues, or put extra effort into team-building activities?
- Introduced initiatives to improve the organization, such as researching new tools to help do your job better/cheaper/faster?
- Do you feel that you could help the organization better if you were in a senior position, maybe with your excellent system design/architecture skills, or your great ability to manage other developers and manage projects? Skills that aren't always in use in more junior positions.
- Have you been working on educating yourself and developing your skills so that the organization will benefit from them?
- Are you so skilled and valuable that other companies are trying to lure you away and a promotion is what will get you to stay (not a great reason but it might count)?
...Or do you just punch the clock from 9 to 5, and you are looking for more money?
You have to give this person a reason to give you a promotion. Think of it this way: if this manager has to justify the promotion of you instead of someone else to their manager, what would you want them to say?