In the book Man's Search for Meaning, Viktor Frankl retold his experiences as a Jewish inmate in Auschwitz, one of the most notorious death camps under the Nazi regime. He recounted the mechanisms that the inmates utilized to cope, to retain their sense of humanity in the direst situations, constantly in risk of torture, loss, and death. Among those tactics, humor remained the most powerful tool to help the helpless prisoners to depersonalize their grief, revitalize their mood, and feel human. The ability to laugh in the face of grief and danger, to momentarily shake the thought of hopelessness and vain, and to forget what tomorrow might bring can tremendously uplift one's physical and mental strength.
Not just in an extreme situation like being in a death camp that humor proves to be useful. In everyday's life, things can get better if we can laugh, finding something to have fun in the most difficult situations.
Be creative, and look at things from a difference perspective, so that they do not seem serious but funny instead.
Search on the Internet for some practical and harmless jokes.
Read comic books, watch comedies, surrounding ourselves with some sources of jokes and humor.
Talk to someone with a good sense of humor, learning from them how they can make fun of almost anything and stay optimistic.
At least, instead of sitting alone, cry, inundate yourselves with negativity, find some things or some reasons to laugh about.