I can't smile
Same here. I feel so embarrassed when people notice my mouth twitching and ask me why I can't smile or don't want to smile. I do want to smile actually just like everybody else, but I find that faking a smile is almost impossible, if not very hard.
My best and once-in-a-life-time smile, at a time when I was feeling great(and less depressed as a result), was a year ago in school when my non-autistic girlfriend(whom I'm proud to say managed to ignore my autistic "quirks.") smiled at me after I complimented her; I dare say that I managed to smile that day.
Thus, I only smile "correctly" when there is a real reason to do so; I don't do any of that fake, smile-at-the-customer crap.
I hate, hate, hate smiling for pictures. I only very recently learned how to smile for photos, and I can only do this when I'm completely relaxed and happy. About a year ago, I was hanging out with my best friend (who also happens to be my cousin that I never get to see), and she asked me why I wouldn't smile for the pictures she was taking of us together. I said, "I can't fake a smile", and everyone looked at me like this: I'm still not completely sure why, but I have a feeling that what I said was rude. I ALWAYS look angry, sad, or confused in pictures because my face won't cooperate. It's very frustrating.
You can practice smiling. There are 2 parts of a smile, one that raises the corners of the lips and one that wrinkles the eyes.
The muscles that raise the lips are located directly beneath the eyes and they make your cheekbones look fatter when you use them. Don't use the cheek muscles that are located horizontal to the corners of your mouth.
The muscles that make your eyes wrinkle are located I think above your cheek bones and to the outside edge of your eye. I've learned to be able to flex them independently but I'm not 100% sure where they are. Sometimes when people are amused their eyes will wrinkle but they won't do a mouth smile. I think this is what makes people think that your eyes sparkle.
You can practice this s**t in the mirror. It takes a lot of practice and you look ret*d if you do it wrong. If you actually work on this then you should always compare your smile to how other people smile to see if it looks the same. When I started practicing it on real people I tried it on people I didn't know and wasn't likely to see again so that I didn't care if I messed up.
I had the advantage that I did smile naturally sometimes and I was able a few times to catch myself when I did it and freeze all my muscles in place so that I could look at myself in the mirror and study it. If you never smile naturally then I think this will be quite difficult. After a long while of practice I was able to make smiling come more naturally. If you smile whenever you feel happy or whenever you're in a social situation that obligates you to smile, then you will start to do it automatically just out of habit, even if it wasn't natural to you before. Now I can do a lot of social cues with minimal conscious effort. I sort of created a "social compartment" in my brain and now I just have to choose to access it and if I do that I find I do things like maintain eye contact, smile, maintain appropriate distance, and other things almost automatically. However I still have social difficulties sometimes because my social compartment is incompatible with other compartment, for example my math compartment. So unless some event is expressly for the purpose of socializing, I'm just about as awkward and anti-social as I ever was. The easiest solution for this is to not care.