Post here any tips you have for entering competitions and I'll add them to this list.
1) Install the http://toolbar.google.com/
google toolbar and enable the "autofill" facility. This will speed up filling in forms because it will automatically fill in most of the information, meaning you can enter more competitions in less time!!
2) Get yourself a free email address at http://www.gmail.com
and enter than when entering competitions just in case one of the companies decides to spam you with newsletters. Remember to check it frequently though in case you win!!
3) Make sure you read the small print, especially the "opt-out" and "opt-in" tick-boxes for the newsletters and "yes, please send me spam!!" opetions!! Make sure you work out which should be ticked and which shouldn't be. They usually say "tick if you do not wish..." or "tick if you do wish..."
Good luck!! I hope you win something nice!! :-D