My little girl smiled at 12 days and hv didn't believe me either - nor did i until she did it the day after and the day after that and on day 16 when she gave me 6 in a row i was convinced - she also smiled for my son and cousin by the time she was 3 weeks and we caught one on camera at this time - she is now 5 weeks and smiles all of the time. She also gives kisses when i make a kissing sound and kiss her she opens her mouth ready!
The best way to get them to smile more is eye contact and coo i find, once they are looking directly into your eyes smile at them and make sure it is exaggerated, she'll soon be smiling wall to wall. You can also look at them directly when cudding and smooth the side of their faces or place your finger on their chin - this also makes them aware that they are smiling and encourages them to use those muscles again, this is best done at their most wakeful time and make sure you react happily again exaggerated when they do smile, to enforce that it is good.
Never forget the first smile - i cried and especially when she smiled at my cousin. We had a freind over today who she has never seen and she smiled constantly - i couldn't believe and nor could my freind!