I really appreciated the post that said "smiling all the time is pretty d*mn close to the ideal".
Well, first off there are several grades of smile. A smile can 'haunt' your lips and be 'hiding' just around the corner, and that is a good expression to wear around town. If you count that a smile, you probably want to have that on 24-7.
But more importantly, just think of what it takes to be able to smile sincerely 24-7. For it not to be fake/contrived, you actually have to be in a good mood and generally feeling good for that much time.
I recently began to solve a big personal problem I had, (porn addiction!), and smiling is easy and automatic now even in public. So I think the question is: can you solve enough of your problems so that you become light-hearted enough to smile anywhere, anytime, in public?
It would be another external advantage that flows from being happy. So this is another 'tip' that advises mimicking the behavior of a person in the optimal, care-free, enjoying-life state.