These questions are tricky because you don't want to scare the hiring manager off by throwing out a number  they can't afford to give you — and you don't want to leave money on the table by choosing an amount that's lower than what they would've offered you.
Knowing your worth in today's marketplace is critical to your success during this process. But a general rule of thumb is that by changing jobs, you can expect a 10% to 20% increase. Much of that depends on your achievements, perceived potential, industry, attitude, the chemistry, and many other factors. The percentage can be higher if, for example, your specialty is in high demand or your current employer pays below market.
Here are some tips to help you negotiate your salary without short-changing yourself:

Offer a range, not a specific number

This gives you wiggle room and allows you to reach for the highest number. You can always drop down from there, as in any negotiation. Also, before going into salary negotiations, know your threshold in advance: What is the lowest salary you'll accept?

Share your research

If you're uncomfortable saying, "I think I'm worth X," refer to your research. This allows you to speak in more objective terms: "My research has shown that this kind of position with my experience is in the range of X."

Be honest

When negotiating salary, make sure you don't embellish your current pay in order to boost your future one. It's a small world, and the truth may be revealed later.

Think in terms of overall compensation

Before you get into specifics, know the entire compensation picture. They may ask you about "salary," but are there bonuses, stock options, vacation time, travel allowances, excellent medical coverage, or use of a company car to consider? Find out before you blurt out a number.

Be reasonable

Don't make an unreasonable salary request. If you shoot for the moon, be prepared to explain why. For instance, if you say, 'I'm now earning N100,000 and I want to be in the N200,000 range,' you'll likely raise eyebrows. But if you quickly back up an aggressive request with understandable circumstances that have kept your salary below market level, you'll get better support. Examples include: a difficult economic environment, downsizing, hiring or salary freezes, cutbacks, or the fact that you receive other forms of compensation and perks.

Don't cave too early

Never throw out a number and then immediately say, "Well, maybe I'm not worth that much," just because the hiring manager looks surprised. If you've done your research and feel confident in your request, stick with it as long as you can. When negotiating salary, you may need to come down in later—but let that process happen more organically.

Say you're willing to negotiate

If you realize that you're out of the employer's ballpark, or feel you're getting limited feedback, you can always say, "I should also mention that I'm flexible when it comes to salary for a great opportunity. Do you also have some flexibility in the compensation for the position?"

Think ahead

Remember that part of your negotiation can be a salary review in three or six months. If you get this in writing and exceed expectations, you may well be at your target salary by year-end.

Pay attention

Like many aspects of job interviewing, you'll exponentially increase your chances of success through active listening, watching for nuances and gauging your responses accordingly. Let the hiring manager be your guide.

If you really don't know or don't care, evade the question

Normally this would be terrible job interview advice. But, if you really don't know or don't care about salary — or you're too uncomfortable answering the question — try something like: "Salary is not my primary criteria. I really place a lot of importance on seeking a challenging, supportive environment where I can make a significant contribution and grow."

Don't settle

Just remember: You get one shot at the salary offer process, so don't settle for something you're really not comfortable with. If you know you're worth more, and not so certain you'd reach your goal salary anytime soon, this might not be the company for you.