The fastest way to lose credibility is for your manager to learn your resignation news through the water cooler conversations. In all situations, your direct manager should be the first person to know, so once your decision is made.
A scheduled meeting will ensure you are both 100% focused on the news, and not distracted by other things. It will also give you the opportunity to discuss the reasons for your decision so your manager understands your rationale, but don't say more than you have to. Be polite, direct, and concise when delivering your resignation, but remember it is very rarely a one-sided conversation.
Chances are, your manager will be keen to find out if there is anything that can be done to change your mind so be ready for a discussion. Try to stick to the facts and remove emotion from the conversation.
Prior to the meeting, you need to know if staying is an option - with the right counter-offer, would you retract your resignation? If you're leaving because of salary, increased benefits, or a promotion, you need to know your walkaway point. But, assess this carefully. Often, the reasons for leaving are deeper than just money so don't let your real reasons become overshadowed by a counter-offer.
You always want to leave under the best possible terms, so make sure you fulfill all of your responsibilities before departing - this includes working out your contractual notice period, which in most instances is 4 weeks. During your notice period, it is essential you continue to work to your full potential, as this is the last memory the business will have of you. Take the time to personally thank other key employees you have worked closely with - you never know when your career paths will cross again.