One thing that will help is to enroll in an obedience class if you haven't done one yet. The other thing I would recommend is that you try the "Nothing In Life Is Free" (NILIF) program. It really seems to work for bossy or aggressive dogs and it even works for shy submissive dogs. Every one has a place in the "pack" and this helps dogs understand that you are the "pack leader" without you having to be harsh. If you do a web search you can find several good articles and suggestions on how to implement this program but here is a pretty good link.
Nothing in Life is Free Good luck,
Teresa