I remember at my second job, I discovered that a co-worker who worked there longer, made less money than I did, and I considered myself underpaid. And that was actually a pretty nice company with decent founders. The co-worker started there as a student I think, and maybe he was never assertive enough in asking for raises, or maybe a bunch of 5% raises still don't amount to much if you start low enough.

Whatever the case, I learned through personal experience that the best way to get a raise is to switch jobs. For some reason, it seems you have more leverage with a prospective employer than with a current one.