Or, just, y'know, use a reliable browser test (not sniffing the user-agent), and if you have an IE 6 user, then replace the page content with a gentle & friendly notice explaining that the site is not compatible with very old web browsers (this is something that a lot of users can understand), along with some helpful links to upgrading IE or downloading Firefox, and a link to a reliable webmaster contact at the bottom.
This is what I do, and it's had zero complaints so far -- on the one or two small sites I've done it for. ;-)
The problem is that the majority of IE6 users are not in a position to upgrade (the ones that are have done so already). There's no point in telling them or bugging them since they are not in control of upgrades.
For example, the NHS (the UK's largest employer and one of the largest in the world) is still using IE6:
No, the best way would be to show a notice in the corner notifying that the website probably won't be optimal using the old browser and ask the user to upgrade.
Don't be intrusive, it's not necessary, and will probably scare away people. Just explain why the website isn't working perfectly.
This is what I do, and it's had zero complaints so far -- on the one or two small sites I've done it for. ;-)