Found an article excerpt that addresses your question pretty nicely:

"Disclosing your salary is shooting yourself in the foot. What is at stake in the negotiation is what you are worth to the company, not a multiplier of what you used to be paid. So, when they ask the inevitable question, be prepared to tell them what you are worth.

Find an equivalent job and its salary range online. Tell the company that you want to agree on a benchmark salary. “Once we agree on that, the rest is easy”, you can say. Present your salary research Now you are ready to negotiate about the details of your experience and their compensation philosophy. When it’s all over, continue to resist the temptation to disclose your salary.

when talking about previous salary, there're a lot of things should be taken into account, including base salary, bonus, pto, 401k, medical insurance and many other benefits...