“Life is short. Smile while you still have teeth.”
Related quotes

“You'll find that life is still worthwhile, if you just smile.”
Lyrics to "Smile", written by John Turner and Geoffrey Claremont Parsons in 1954, the music of which was composed by Chaplin in 1936. - "Smile" music, as used in Modern Times (1936) http://www.youtube.com/watch?v=Ps6ck1ejoAw - "Smile" tribute to Chaplin, as sung by Michael Jackson http://www.youtube.com/watch?v=iu-rLA4POkI
Misattributed
Context: Smile though your heart is aching
Smile even though its breaking
When there are clouds in the sky, you'll get by
If you smile with your fear and sorrow
Smile and maybe tomorrow
You'll find that life is still worthwhile If you just
Light up your face with gladness
Hide every trace of sadness
Although a tear may be ever so near
That's the time you must keep on trying
Smile, what's the use of crying?
You'll find that life is still worthwhile.

“A sociable smile is nothing but teeth.”
Sometimes misquoted as "A sociable smile is nothing but a mouth full of teeth."
Desolation Angels (1965)

“Behind every smile there's teeth.”
"Model's Web rants pined for love" in Daily News (New York, 29 June 2009) http://www.nydailynews.com/news/2008/06/28/2008-06-28_models_web_rants_pined_for_love.html

“Then take, good sir, your pleasure while you may;
With life so short 'twere wrong to lose a day.”
Dum licet, in rebus jucundis vive beatus;
Vive memor quam sis aevi brevis.
Book II, satire viii, line 96 (trans. Conington)
Satires (c. 35 BC and 30 BC)
“Then take, good sir, your pleasure while you may;
With life so short 'twere wrong to lose a day.”
Dum licet, in rebus jucundis vive beatus;
Vive memor quam sis aevi brevis.
Book II, satire viii, line 96 (trans. Conington)
Translations, The Satires, Epistles, and Art of Poetry of Horace (1869), Satires