One thing that is not mentioned is stressing exactly how valuable your skills have been to previous employers. Stating exactly how you saved X amount of money or added X amount of new revenue is a really powerful reason that some get paid more then others. patio11 has a really great post about this: http://www.kalzumeus.com/2011/10/28/dont-call-yourself-a-pro...

Remember businesses are about making money. The way to get paid more is to demonstrate you can make them more money.