To me, some of the simple solution to cheer someone up is through hugs, love, and sharing. Most times when I share my sad feelings with some friends, their hugs often act as soothing balms to calm my frayed nerves. These friends often make me understand that they are with me in whatever I might be passing through and reassure me that the situation is not as hopeless as I think. They make me see their readiness to lend their supports to me whenever I encounter a situation I can't go through alone. So, give your friends some friendly hugs whenever you see them unhappy, spend some quality time with them to know what exactly is eating them up. Reassure them of your unflinching support during their ordeals. Encourage them to look at the bright side of life and hope for the best.
While you are with them, buy them some nice gifts, especially those things they have always wished to have. You can also prepare them their favorite meals. I often forget most of my hard feelings after I have had a good meal. At such times, some humor can go a long way to heal broken hearts. Laughter is always a good remedy for sad situations, so do whatever you can to make a friend giggle and forget his sorrows.
Ask some open-ended questions and engage in some kind of brainstorming to enable your grieving friend have a better view of their difficulties and the best solutions for them. Don't forget to encourage them to remain positive and hope for the best. You can be creative and set some goals to foster a feeling of hope and optimism.