How to negotiate salary during your final interview?

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I've been worked for several years and ever worked in 6 different companies.I feel it's easy for me to get through the interview and also got good impressions from interviewer but,I really don't know how to negotiate a proper salary according my work and personal values,and most times my salary lower than my coworkers who have similar work abilities,even one of my interviewers ever said to me,why you didn't ask for a better salary? I feel quite embarrasing to negotiate salary with boss,i always worry about if i asked too much?So most times i only ask a salary under average...Could you tell me how to negotiate salary during interview?