Back before I became a consultant after about 2-3 years at whatever my current job was I'd start taking a couple interviews every 6 months. This allowed me to gauge the market for what skill sets/tools were being used in my area and also what salaries are offered. I never considered using this information as leverage though, I just left when I got a better or more interesting job offer. Now I consult so I am interviewing every 6-18 months anyways.