'Always look on the bright side of life'.

Or perhaps, Keep on the Sunny Side:

There's a dark and a troubled side of life
There's a bright and a sunnyside, too
'though we meet with the darkness and strife
The sunnyside we also may view.
Keep on the sunnyside
Always on the sunnyside
Keep on the sunnyside of life
It will help us everyday
It will brighten all our way
If we keep on the sunnyside of life...

That's pretty good advice too. (Seriously!)