Know when the sweepstakes/contest ends. You waste time entering when the expiration date has past.
Always read the Official Rules. You don't want to disqualify yourself if you enter more than once and you are only required to enter one time. Some sweepstakes/contests allow you to enter once per day or once per e-mail address, or once per household. Read Read Read!!!!!!!
Know the sites you enter. Not all sweepstakes/contests are legit. If they ask for personal information, other than your name, address, date of birth, etc., do not use their site(s). I for one don't enter anything that is sponsored by third party groups. Some site will say "click here to claim your prize before you've even entered." Claim means you must do (buy, etc.) to obtain your prize. Exit this site!! Sweepstakes/Contests are ALL FREE to enter.
Have a valid email address, home address, and phone number. You don't want to win a contest and the sponsors of the sweepstakes can't reach you or maybe they are able to reach you via e-mail, but you don't have a sufficient address or phone number. Sponsors usually give you a specified amount of time to respond once you have been notified of your winnings; be prompt, reply, and do whatever requirements are given. Other times, sponsors just mail it to you without notifying you.
Always read the requirements. There may be additions needed, such as a photo upload, essay, or age requirements. In addition, pay attention to sweepstakes that require you to accept magazine subscriptions. (I had to learn the hard way. It can be difficult to locate the "subscription cancellation" link on their websites). Other companies, in addition to the entry form, may want also want you to subscribe to their newsletter via email to enter. This is okay to do and is free (based on my experience). Just make sure to check your email to confirm your subscription. Follow directions and read the fine print.