Well that only applies to posts on the forums.
I think the way AvE (and some other youtubers) has dealt with it is decent, and thats just by picking random comments. There are websites to do so and they even take into consideration only one entry per person.
Stipulations could be made for subscribers only as well if thats the kind of route you want to go.
@wendell Far be it from me to suggest how to run the competition though. I just like bitching.