Humor is a strange thing, have you ever thought what makes something funny?
A great example is illustrated below, how do you turn this Horror Halloween costume into a Funny Halloween costume?
From Horror Costume
...To Funny Costume
Now I don't know if it is just my "sick" sense of humor, but that addition of a bit of blood and the fact that the guy is hitch hiking and has a sign makes it funny - but as soon as you try and explain it... it is no longer funny!
Anyway if you want to see some more costumes to make you smile, check out this funny costumes page.