1. You want to be careful to avoid online contest scams. Anyone who asks you to pay, asks for your social security or banking information, or tells you that you’ve won a contest that you don’t remember entering is probably a scam.
2. There might be hidden costs. Realize that depending on where you live, you might have to pay taxes on your big wins. There have been cases of contest winners taking home free cars, and then realizing they can’t afford to pay the taxes on their prize or afford the insurance. 
3. You might get a lot of emails. If you enter a ton of online sweepstakes, your email box could potentially get flooded with contest-related information. You might want to consider creating a separate email account for your contest mail.
4. There are third-party clients such as Roboform that can help you fill out contest forms and submit your entries quickly and efficiently so that you don’t spend too much time entering each contest.
5. The more you enter, the better your chances to win. If you are serious about winning big, you should consider entering several contests every day.
6. There are many sites that aggregate contests and sweepstakes from all over the web, and these sites can be a great resource to you.

Above all, remember that entering online sweepstakes is a hobby and that it should be fun.