Well... that's actually a VERY COMPLEX question, with NO straightforward easy answer.
Depending upon the situation, you could begin by doing this:
1) First, tell your friend that you noticed he/she is really feeling down.
2) Next, you can tell your friend that you actually went out of your way to post an Internet forum discussion question (such as this particular question on Reddit) about how to cheer up your friend.
That will, demonstrate to your friend that you really care about them, since you went to the trouble of thinking about their situation, and you are actually seeking help from a large online community.
Even if they don't say it out loud, they will like the fact that there is someone else in the world that cares this much about them.
3) Next, you can also tell your friend that you are aware that trying to go out of your way to "cheer" someone up can often be the WRONG thing to do!
Sometimes when we are going through something very traumatic, or difficult in life, the LAST THING we want is a bunch of people trying to "cheer" us up, or make us "snap" out of it.
INSTEAD: sometimes we have to go through a period of sadness, melancholy, and depression.
Such moments are COMPLETELY NORMAL, and a PART OF LIFE.
4) At the same time, after telling your friend you are aware that trying to "cheer" someone up is sometimes the wrong thing to do (since we all have to sometimes go through periods of melancholy/depression) you can make a few little jokes, if you think it is appropriate, and actually do have a couple of funny jokes.
When making the joke, you could then just say:
"Sorry... I couldn't help trying to make a joke or two to cheer you up, even though I know it's normal/important for you to go through this down time."
But don't overdo it on the jokes to excess, as that can get really annoying!
5) Further... keep in mind that some people subconsciously learn they can get extra attention from friends/family by being depressed or melancholy.
So if this situation with your friend keeps happening over/over... and you begin to feel they are dragging you down, and sapping away your energy/time... then in the end you can NOT let them drain too much from your own life, over a longer period of time.
You have your own life to live as well.
Eventually it might be time to move on, and find a new friend.
6) FINALLY... keep in mind that some types of depression/melancholy are the result of a bio-chemical imbalance within the human brain, or an actual physical/structural issue with the human brain (related to the mapping/shaping of physical neural networks in the brain).
If it is a physical/biochemical situation within your friend's brain, then NO AMOUNT of trying to cheer them up will help.
You can NOT just magically whisk away a biochemical issue, or physical/structural issue in someone's brain, just by making a few clever jokes, or drinking a few glasses of alcohol, or going out for a fun night on the town, or throwing a ton of money at the person.
In such cases, the physical or biochemical issue in their brain will remain, no matter how much energy/time you put into trying to "cheer" them up, and instead that person MIGHT need the help of medicine.
In those cases you will likely NOT be equipped to help them. (If that turns out to be the case, then again, you MUST move on with your own life, and don't let them drag you down.)
In those cases they will need a professional to help them. (Not you.)
In short... it's perfectly NORMAL to go through a relatively long period of melancholy or depression in life... During those normal instances, it is important you are there for them as a friend.
And of course, in many of those instances, sometimes a person does NOT want you to go out of your way to "cheer them up" excessively... but rather they might just want to have a good friend to be there for them, as they go through the issue.
But sometimes the problem can be far greater than we suspect, and can be biochemical or physical-medical in nature... or a very complex psychological issue that is beyond the ability of any friend to help with.