Don't hold ad-click contests, beg, or even give big hints that people should click on your ads. They can ban you if they catch you begging for clicks anywhere on the Web, including pages that are totally unrelated to your AdSense pages.
Google also forbids labeling your ads with language stronger than "sponsored links." This is really for everyone's benefit. Pages that beg for clicks are usually not great reads, and pity clicks don't help the advertisers.
Note: It's fine to have contests on your Web site that aren't related to ad clicking or other rule breaking, such as "best photo" contests.