This is, in my experience, the biggest aspect of the whole process. If you go into a raise negotitation undervaluing yourself, you are always going to come out short. By looking at my role, how I help the company, and what it would take for them to replace my skillset (including training and learning company policies/procedures etc) I have massively improved my confidence in this area and been able to consistently negotiate much bigger raises.

Most companies would much rather throw $10-20k at a small problem to make it go away, instead of going through the hassle of a new hire and everything that goes with it.