How to prevent a user from registering multiple accounts to vote themselves up

Question

This question isn't about preventing a single user from registering multiple accounts. It is assuming that registering multiple accounts cannot be prevented, but what we want to avoid/detect is when the user is using the accounts to vote for himself. This would apply to sites like SO or digg, where a user creates a 2nd account solely for the purpose of of voting up their 1st account.

Any ideas how gaming the system in that way can be prevented/detected? Any insights how SO does it? :)

Answer 1

The creators of the site wrote about Vote Fraud and You.