Good question! I have a BS in Psychology with a minor in Biology---during undergrad, I was considering a career in experimental, neuropsychology.
However, I jumped right into Marketing (computer-tech) and never looked back. I found my background (and passion) for psychology and other social sciences have been instrumental in accelerating my my career growth.
I have found opportunities and success in Product Marketing, Brand Marketing, and I currently lead Social Media Marketing for Norton (by Symantec).
These "6 Principle of Persuasion", by Dr. Robert Cialdini , are great examples of how psychology can be used as a tool for Marketers. http://www.influenceatwork.com/wp-content/uploads/2012/02/E_Brand_principles.pdf
Principle #1: Reciprocation Principle #2: Social Proof Principle #3: Commitment and Consistency Principle #4: Liking Principle #5: Authority Principle #6: Scarcity
These tools and principles don't just apply to selling products or services, but can also be used to promote YOURSELF and accelerate your personal and professional growth.
Coincidentally, I am at a Business conference at this very moment and a Keynote presentation tomorrow is titled "Psychology-based Marketing"!
I hope this helped :)