You don't ask, you don't get. Simple as.I'm between jobs and added 8% to my salary expectation. Whether I get that or not is another matter.
I am a recruiter and deal with this as a job and people are always trying it, often it works. I've seen people double their salaries by changing jobs (£40-£80k) and its common to see people go from £90k-120k or £80k-100k as well. Companies this year that are not seriously evaluating their salaries and paying their staff more will have the most attrition.