I have been here since june or july. In total I've only won two giveaways. Having many good quality reviews with nice pictures gives you a higher chance of winning a giveaway.
I have about 17 reviews and from what I've noticed is that you need more reviews than that. Therefore I only win the big giveaways that are for 3000 participants etc.
Another user has shown you a good chart for getting picky points. You get the most points by reviews with pictures, logging in daily and being active in the threads will also give you picky points, that you can exchange for giveaways.