It’s been pointed out to me that I don’t smile very often. It’s not that I’m generally unhappy it’s just that the natural expression on my face is generally very neutral and has even been interpreted as a frown.
So, I’m wondering how exactly does one learn how to smile and have it feel natural and not like a cheesy grin? Do I just keep doing it and eventually it will feel normal and not idiotic? And does anyone else have this issue or am I just more of a freak than I originally suspected. 
Do people keep telling you to smile? Tell them to eff off. Doing that always puts a smile on my face.
I have a friend who is like you - her natural expression is a bit glum. She tends to look sad if she isn’t actively smiling. That’s just how her face is, and her friends know not to assume that her dog just died or something.
I apparantly also look mildly pissed off if I’m not actively smiling. I try not to worry about it. 
Ha. That made me smile too.
Me as well. 
Tell them that you’re smiling on the inside.
Well, it’s either that or bowel cancer.
Apparently, my natural expression is one of supreme boredom. In my defense, I’m not always bored…
After two different bosses brought this to my attention, I made a concerted effort to look engaged and interested. I felt like a complete doofus but it apparently worked. It still doesn’t come naturally though.
Smile with your eyes. Arching your eyebrows should make you mouth follow. Relax the jaw, let it hang loose a little. Widen your eyes, cock your head slightly to one side. Some of these things are indications of a real smile, others contra-indicate a fake smile.
You’re in good company. Queen Elizabeth II has been quoted as lamenting the fact that her natural expression makes people think she’s being grumpy. (I can’t find the exact quote right now.)
Edit: here it is, right on the SDMB!
Yeah, my wife regularly asks me if I’m sad - or even tells me I look sad. Most of the time I’m not.
I seem to recall a study about the “twinkle” in the eyes of people who are naturally smiling. It has something to do with an involuntary crinkling of the eyes around the corners that makes the smile appear more genuine to us.
In other words, don’t bother trying to fake a grin. Smile when you mean it.
Sometimes it is useful to be able to pull off a fake smile, though. I agree that crinkles around the eyes read as more genuine. A tiny squint can mimic that effect. Also, a bit of scrunching up the nose also helps create the illusion.
Yes, it’s called the Duchenne smile. It works – if I have to smile on cue, I make sure to crinkle my eyes and it does look a whole lot more natural.
You can fake it. Wait for a time you’re really smiling and pay attention to what the muscles around your eyes are doing and how they feel. You can isolate that and fake the crinkling. Doing so usually makes the corners of your mouth turn up a bit as well, adding to the effect.
I actually practice it in a mirror. When I don’t take that out into real life, people accuse me of being angry. Not too long ago, I had several people at a lunch counter say “Woah, dude, don’t kill us!” And these are people that know me and how cheerful I normally am.
Apparently my natural expression is one of an axe murderer, so I have to consciously choose a happy face. It makes life much easier!
That was a good tip, I tried it and it works. With a little practice I think it could work very well. I take horrible photos because of fake smiles. Naturally I look more like a mug shot. I envy those who can turn the smile on like a light bulb.
Practicing in front of a mirror really does help. I was in a situation where I was taking a lot of photos- something that I don’t normally do well. After a few sessions in front of the mirror, I managed to get a passable on demand photo-ready smile. Do note, however, that a photo smile and a real smile are going to be a little different. A real smile will have more scrunching around the eyes, and can be a little less neat.
One tip- wait a beat before busting out the smile. If you smile immediately upon seeing someone, it can come off as fake. It’s a little more natural and special feeling to see them, take a moment, and then let the smile warmly spread.
I laughed at the description on that page of the “Pan Am smile.” 
While you should be able to have the expression you want, it is good to have a good fake smile ready for interviews and other places where you want to appear warm and kind.
My face falls into a natural pout, but I smile all the time, so i don’t really have this problem. But, you can practice in the mirror as other people have said. Just make sure your smile doesn’t scare small children.
What if I want to be genuinely warm and kind?
I know a lot of people get pissed off when others tell them to smile, but it really does help. If you act outwardly happy, your emotions will follow. There was even a really interesting study about this.