Prevent double voting

You can combine these two methods:

• Add a cookie to prevent multiple votes from the same machine
• Log IP addresses and prevent voting more than a set number of times from the same address (for example, 5 times the same hour).

This will make it possible for multiple persons to vote from the same network but still prevent excessive cheating.

You may also make it harder to build a voting bot by adding some hidden form field with a token that must be included in the vote, and/or use Ajax for the voting. I know it's relatively easy to build a bot anyway but most cheaters aren't that smart.