Use QR Codes
An idea to get users to easily browse to the relevant private link for a specific poll is to make a QR code display as part of the poll instructions. That way the the user can easily take a picture of the slide and get the direct link on their phone.
Then they also don't need to enter a complicated code.
For example see here:
Just an idea.
Great idea! And currently possible via numerous existing URL to QR code services out there on the web.
You could even take this a step further and tie the QR code to individual voting options (Hint: Right click on the options of a poll's private link to find the voting URL.)