When you're ready to quit

So either you've found a better job or you can't stand another minute in this one.  Read this and then go talk to your boss.

• Don't feel guilty - Whatever your reason for quitting - but especially if you are leaving for a better job - don't feel guilty about it.  Even if you are leaving a pretty good job with an understanding and supportive boss, you have to do what's best for you.  You can be sad or a little sorry to quit, but don't let the feelings of your boss or co-workers factor into your decision.  Do what's right for you.  A good boss will be sorry to see you go if you've been a valuable employee, but you did communicate your dissatisfaction, didn't you?  Or maybe you got a better offer out of the blue.  I've had plenty of good people quit on me and I learned quickly to see the situation from their point of view and wish them well.  Yes, it's disruptive, but part of management is dealing with employee turnover.  One thing I can guarantee you, the company will not be thinking about your feelings if it has to fire you.
• Give appropriate notice - At least two weeks notice is common for most lower level jobs.  You should be prepared to turn over your projects and train your replacement, if one can be found before you leave.  If you have the time before you announce you are leaving, maybe you can start the process of tying up loose ends and documenting the status of tasks you are working on.  Also, be aware that some companies get pissed off when employees leave (especially if they are going to a competitor) and will have you leave immediately.  If you think this might happen be sure to prepare your desk and files for a hasty departure.
• Don't unload on your boss or the company - I'm a believer in the 'don't burn bridges' philosophy.  If your boss has been a real pain it's enough to say (to him) "You and I just never got along and I've found a place which can use my skills better."  Like breaking up with your girlfriend or boyfriend, you might want to try "It's not you, it's me" if you don't want to talk about the real problem.  Resist the temptation to boast or brag about the new job.  If you do want to unload, you can do it during the exit interview with HR.  By the way, if your boss has been a good person but you've found a better job, tell them.  They will appreciate hearing it.
• Handling a counter offer - So in some cases if you are a really valuable employee and you're leaving for a better job, the company may respond with a counteroffer.  "We didn't know you were looking for more money.  Would you change your mind if we pay you an additional $3000 a year?"  To me, this is insulting.  It's like saying we didn't realize how valuable you were until someone else pointed it out.  Particularly if you've been communicating your desire for more money and more responsibility all along.  A boss who waits until an employee has a better offer to come up with a raise isn't doing her job.  I suggest you thank them for the offer but happily go off to the new job. 
• The exit interview - Larger companies will schedule an interview for a departing employee to remind them of obligations of confidentiality, handle paperwork, and talk to you about your reasons for leaving.  Employee turnover is disruptive and expensive so most companies try to minimize it.  If your boss has been a challenge, or the job was a dead-end, or whatever the reason you had for leaving, you can communicate it during the interview if you want.  You don't have to, but you may be doing the remaining employees a favor.   Be factual and honest, not emotional.  Particularly if the problem has been your boss, you should be prepared to describe the problems you had, how you attempted to rectify them, and what happened.  The HR person should listen and take notes and not be defensive.