Especially now that we are in the month of Adar, when we are told that the standard "Serve G-d with joy" is not enough for this month, we must intensify our joy.
But what are we to do when sometimes it's just too hard, sometimes we just don't feel we really have much to smile about??
The Tzemach Tzedek (the third Lubavitcher Rebbe) suggested to one of his followers who was having a hard time attaining a sufficient level of joy, to act as if he has a full and joyous heart, to show joyous mannerisms even if that is not how he really feels at the moment. And ultimately, he will feel this way in actuality.
In other words, "fake it, till you make it".
In a similar vein, a follower came to the Alter Rebbe (the first Lubavitcher Rebbe) asking how he could help his fellow Jew who pretended to be pious, but was actually quite a sinner.
The Alter Rebbe said to him: "The Mishna says that a person who pretends to be a pauper but is not will ultimately become a pauper. So, too, this man who pretends to be pious but is not should ultimately become pious!"
As we see here, the initial step to being happy is to pretend we are happy even if we are not. Eventually, the play-acting will no longer be acting but actual.
Judaism teaches "Receive all people happily" and "Receive them with a cheerful countenance." Even if we aren't inwardly, genuinely happy to see someone, at least we should greet them with a cheerful countenance, an external expression of joy.
As a great sage taught us, "Even if your heart does not rejoice when someone visits you, you can pretend to be cheerful when he arrives."
So, be happy! It's Adar! And if you don't feel happy, fake it until you do!