Smile as much as you gosh darn possibly can! Frowns are expensive for your body - not only do they use more muscles, but they generate stress, negative energy towards others, and then require more energy to reverse all of those things! When I see other people frown, I get more anxious just being around them.

Here's my rule of thumb (and yes, I do actually think before frowning...most of the time;)
How bad is this? Will I still be alive and not even changed that much from now? (Usually this answer is yes!) Then I choose not to frown.

Smile a little smile for me....