Face-to-face is the only way. Don't offer any constructive criticism. Don't tell colleagues until you've told your boss. Since the letter thing has been beaten to death (it's irrelevant, it's a procedural piece for HR dorks) I will stress one thing: if you're leaving, there are some bosses and/or company cultures that now view you as an enemy. Depending on your industry or your title, it's not unreasonable to consider that they may walk you that day. So prepare for that in terms of transferring files, deleting your porn, throwing away the scribblings you made of the boss being forcibly sodomized on the back of your Sexual Harassment Training binders. Also, time it correctly. I don't know how many idiots I've seen tender a resignation the day before bonus eligibility and miss out on several thousand dollars. I saw one guy tender a resignation that was two days shy of his being vested in the company pension plan ($25,000 he could have rolled to a Roth IRA or paid the taxes and taken as cash). The boss would have helped him out in advising him to take a later date, but the employee was such a dick in his resignation, the boss just smirked and accepted it.
