"Always be joyful." - 1 Thessalonians 5:16
It sounds simple, doesn't it? Just be full of joy all the time. Try it. Frustrated? This would be a frustrating adage if it were up to us to provide the joy. We can't give ourselves joy though. It has to come from someone bigger than us and all our circumstances. If you find yourself lacking joy today, go to the one who can give it freely.