When to Negotiate
There is a relatively small window of opportunity to negotiate during the hiring process. It is between the time the company makes you a bona fide job offer and the time you accept that offer.
Any earlier, and it’s all hypothetical. And you may be shooting yourself in the foot by asking for more too early.
Any later, and it’s too late. The company isn’t likely to give you more AFTER you’ve said yes.
A bona fide job offer is typically in writing, or at least a formal verbal offer of employment. If you are unsure, you can ask “Are you officially offering me the position?”

What are My Options?
You have three options when a job offer is made:
-Accept the offer as-is
-Reject the offer
-Negotiate the offer

Should I Negotiate?
If you are completely, totally happy with every aspect of the offer, then you don’t have to negotiate.
I highly recommend that you ask for a few days to a week to consider the offer before accepting. This gives you time to thoroughly evaluate the offer and keeps you from looking “desperate.”
 
When is “No” the Appropriate Response?
-When you have already accepted another offer
-When you know the company or job is in direct conflict with your values and career goals
-When you know the salary or other aspects of the job don’t mesh with your needs (i.e. doesn’t pay enough for you to live on, extensive travel for a new mom)
-When you hear or read considerable intel that the company is financially unstable, your prospective boss is very difficult to work for, or the work environment is toxic.
NOTE: Always do this politely so you aren’t burning your bridges.

How do I Negotiate?
Let’s assume you are in the window for negotiations.
If salary is the issue:
-Research salaries for the same or similar jobs in the same geographic region
-Send a letter of negotiation with a counter-proposal for your salary, and provide statistics from the research you’ve conducted

Tips for negotiating from Harvard Business Review, written by Deepak Malhotra:
Don’t underestimate the importance of likability.
“People are going to fight for you only if they like you. Anything you do in a negotiation that makes you less likable reduces the chances that the other side will work to get you a better offer. This is about more than being polite; it’s about managing some inevitable tensions in negotiation, such as asking for what you deserve without seeming greedy, pointing out deficiencies in the offer without seeming petty, and being persistent without being a nuisance. Negotiators can typically avoid these pitfalls by evaluating how others are likely to perceive their approach.”
 
Help them understand why you deserve what you’re requesting.
“They also have to believe you’re worth the offer you want. Never let your proposal speak for itself—always tell the story that goes with it. Don’t just state your desire (a 15% higher salary, say, or permission to work from home one day a week); explain precisely why it’s justified (the reasons you deserve more money than others they may have hired, or that your children come home from school early on Fridays). If you have no justification for a demand, it may be unwise to make it. Again, keep in mind the inherent tension between being likable and explaining why you deserve more: Suggesting that you’re especially valuable can make you sound arrogant if you haven’t thought through how best to communicate the message.”

Make it clear they can get you.
People won’t want to expend political or social capital to get approval for a strong or improved offer if they suspect that at the end of the day, you’re still going to say, “No, thanks.” If you intend to negotiate for a better package, make it clear that you’re serious about working for this employer. If you’re planning to mention all the options you have as leverage, you should balance that by saying why—or under what conditions—you would be happy to forgo those options and accept an offer.

Understand the person across the table.
Companies