I will constantly get people asking me "What's wrong?" or saying "Cheer up" or "You look depressed" because I don't spend the entire day grinning for no reason. I speak with a monotone when dealing with general communication, so that doesn't help, either. Just because I don't look "happy" doesn't mean I feel bad.
When engaged in a conversation among friends, I have a tendency to do exactly the opposite, because I am no longer dealing so much with the conveyance of information as with actually having a good time. Several people have told me I make a lot of strange facial expressions and I'm quite animated, which I am, around people I know I can be.
Many of the mannerisms I adopt in casual conversation are revisions of things I have seen in movies; Jim Carrey, Cary Grant, Harold Lloyd, and Johnny Depp movies, generally. They all obviously made studies of human interactions and body language, and were widely successful at exaggerating it successfully in a more of less believable way. By recognizing their exaggerated reactions, it helped me to be able to recognize the more subtle reactions of everyday people, and be able to know when I should employ them myself. Of course, mine are slightly "over" usual body language / facial expressions, but that's just to amuse myself.
Humor is a largely motivating factor for me; if I didn't employ body language that amused me, I wouldn't bother at all because it would just be a waste of time.
What is your motivating factor? Just wanting to do it isn't really enough if there's no specific reward. You need to find gestures / expressions that actually interest and appeal to you, or you won't be able to copy them.