Rule Nr. 1: There are more scams than you can shake your fist at. Check whether the contest that has caught your fancy comes from a reputable corporation. Yes, these do put together such events with alarming regularity.
The reason? Marketing research, pure and simple.
Here's a warning sign, though: these companies are wary of sending out unsolicited e-mails. They don't ask for your banking information, or for your credit card number, either. And the rule of thumb that anything that sounds too good to be true is too good to be true, applies, too.
Rule Nr. 2: There are web sites out there that will help you keep up with sundry competitions, filtering those that aren't kosher in the process, too.
Here's a selection, courtesy the abovementioned Franklin Prosperity Report:
http://www.winprizesonline.com/,
http://www.prizey.net/,
www.sweepsgoat.com,
http://www.online-sweepstakes.com/,
www.sweepsadvantage.com.