When I come across someone negative...I try not to take it personally. I assume that person treats everyone the same way and that I'm simply another example of that person's poor character.
If I'm having difficulty communicating with someone, I attempt to determine the intent on what they are conveying. Is it positive or negative...is this person trying to help me or hurt me? Often times we'll find that negative people are projecting their own insecurities.
The only thing we can control is our perceptions and reactions...we can sink to a lower level or rise above.