If you are using other limitations, such as one vote per user, I suggest requiring users to login to vote. While cookies make a best effort at keeping folks from gaming the system, various workarounds (such as using incognito windows) can cause confusion and uncertainty with the final tallies. This also means they will need to create an account, so streamlining the information required in the user profile is key if a quick process is important.
I also suggest enabling social sharing to drive more traffic to the voting page, while raising awareness of your program. Social sharing can give folks an easy means to get their friends and co-workers involved in the process.
To learn more about how to enable social sharing, and the public voting process, check out this help article124.