I've had several questions lately about adding Polls and/or Surveys to your WordPress blog in order to do a 'contest'. And in a timely turn, today I came across an excellent article with suggestions on how to do this using plugins or add ons that take very little tweaking on your part:
Remember as with any add-in application, please contact the plugin developer if you have any problems or need help with it. And if the plugin is free, plying the developer with money helps to get a response, LOL.
And as with any plugin you add, conflicts sometimes occur, so you need to be thoughtful not not every plugin will work on every site. (That was my 'buzz-kill' disclaimer)