I used to buy a magazine that was full of competitions and I religiously filled many of them in each week, and not once did I ever get a prize.
Eventually I got sick of it and stopped buying the magazine.
I very rarely enter one of the supermarket competitions -in fact I do not remember when I last did.
I have an acquaintance who enters as many as she can find and she reckons she wins things, but it is very time consuming anyway, and not worth it most of the time.