Read this section. Note the paragraph on "better positioning", which says that your past salary signals to employers your level, title, and responsibilities. To obtain a higher salary, you need to position yourself so employers do not question your abilities because you have not received a high salary in the past. However, sometimes it makes sense to take a lower salary.
The article responds to similar questions, such as:
There may be other benefits worth negotiating besides salary. This reading helps you to consider several scenarios and appropriate approaches to formulate your initial position.