In reply to mwr72:
As said above, I have always told my boss personally and, after the conversation, handed him/her a very short letter setting out the fact that I am resigning as of today's date subject to whatever notice period is required or otherwise agreed between you.
I think it would be normal, unless you felt otherwise, to say something brief about having enjoyed your time with the company. Personally, I wouldn't say anything more in that letter - which will go on your HR file.
Whatever you say to your boss is less formal though probably more important and may be remembered/forgotten etc. I recall one particular instance where I handed my notice in to move up to the Lakes to work/climb and my then boss said I'd get bored and he'd call me in a year and he'd be very happy for me to come back. That's pretty much what happened, sadly perhaps.
Nowadays I'd have to write to myself, which makes resigning oddly more difficult. I've thought about it though!
Post edited at 16:27