Resigning

Don't resign until you've got your new job offer in writing

What are the pros and cons of your current job
Is the new job the right move?
Why did you look to change jobs in the first place? Why were you unhappy? Will these circumstances change?
How would you feel if you were offered a promotion or a pay rise?
Don't be swayed by friends and family encouraging a lack of action
Don't be put off by imagining that it won't be the same without you

Remember your career is the important thing; the company will survive without you

Include to whom it is addressed, the notice period, when it is effective from and your signature
If you are leaving on good terms, concise sentiment goes a long way. Don't burn any bridges. Thank your boss for opportunities and express regret at leaving and a hope to keep in touch.
If you are leaving because of a specific problem, don't elaborate. Keep it simple, don't document your anger or frustration, just keep your letter polite and to the point.

You may feel guilty or anxious or that you have let people down..
You may be sad about leaving friends

Remember why you are leaving and the positives about your new job.

Most people (including your boss!) have been in this situation. You won't be the first to resign at your company and you won't be the last.

Find the right moment for you
Keep it confidential before you tell your boss
Rehearse your reasons for leaving - you are bound to be asked
Don't worry about the reaction, be polite and take it on the chin. It's almost never as bad as you imagine it will be
If there is a handover period cooperate
Stick to your guns. Don't be talked out of it.
Don't be offended if your boss doesn't try to talk you out of it - remember, it is your choice to leave and if you've followed the advice above, you are doing it for the right reasons and for your own satisfaction.