The first way to win your magical elf-made free prizes is 'skill' with "click on the monkey/smiley/fucking thing" banners. I hate to break it to you, seat-warmer, but your ability to move a mouse and click it is NOT the unique skill, honed by years of hard work, that is about to start paying off for you. There are no olympic games in mouse mastery, no bling or ho's for cursor movers, and no fabulous cash prizes for outwitting a looping two second animation. Hitting the moving monkey is like successfully placing a CD in a toaster - quite easy to do and you're going to look stupid for expecting a reward.
The second way is to be as lucky as me - I've been the 1,000,000th visitor to seventeen sites just today, some of them several times in a row. I could buy a lotto ticket, wrap it around some dice thrown into a roulette wheel and win all three. I'm less likely than a 50 cent shout out to Vanilla Ice. With my amazing anti-statistical-likelihood aura it's only a matter of time before I start levitating or spontaneously burst into flames, and it's this knowledge that every second could be my last which prevents me from claiming the warehouse of wonderful gifts with my name on it. It's only fair I leave them to somebody who'll live longer, like the next person to be 1,000,000th visitor. Probably you, so you're welcome.
One thing all these ads have in common is legalese fine print which, like mobile-phone cancer and minesweepers wrist, is a disease of modern society. The usual litigious strategy of burying phrases like "We're going to take everything you own at ten million per cent interest then bend you over without lubrication" inside ten page hyper-syllabic documents doesn't work in banner ads, which is why in some the entire second half is dedicated to disclaiming the first half, while others have realized that a simple "*" means you can say anything you want. For those of you too busy claiming your wonderful unicorn-delivered internet pots of leprechaun gold, here's a handy translation of every prize-winning banner ever:
"You have won a FREE* thing!
(*Fuck you, moron.)"