Many Professionals really like their jobs. Get a good feel for what people with similar skills and similar years of experience are getting paid. Most of this information is commonly available on the Internet.
Some people are just plain good negotiators.
However, salary negotiation is different. When the salary negotiations are over, you'll probably have to work with the person with whom you're negotiating. Moreover, your future success may depend on that person. So, while you want to negotiate the best possible deal, you need to do so in a way that doesn't damage your image. Decide on the minimum raise amount you are willing to accept.