Make it a toothy grin.

When you smile, don’t hold back.  Show as many teeth as you can.  See if the observer can count at least half your teeth when you beam at them.  A smile is infectious.  But rather than a nasty virus, you’re spreading mirth and merriment.*  And if you can smile even in the worst of circumstances, then you’ve truly mastered the secret to a happier life.

*Even if you have poor dental hygiene, people will still return your smile, though they may back up a step or two.

More SuperOptimism:

Look outside yourself.
Stop and smell everything.  
This could be the day you meet someone new.