One way would be the same thing you do with "forgot my password" links. You enter your email address, a link with a random activation key gets mailed to you, click on the link in your email client and get logged back in. The activation key would have a limited valid span, say 30 mins, so even if anyone got hold of it it would be useless.

The disadvantages are a) your email can be hacked and b) it's a bit inconvenient for the user. Also you'd need SSL to protect the key, which would be in the URL.