First, a good ending is always inherent in the opening. A predictable or cliched ending just means you didn't take the hard left turn a good short story needs. It's good to see a predictable or cliched ending coming, and then turn it on its head. This is what separates a story that sells from one that doesn't.
There are many ways to write a short story, and many structures to use, but I think all short stories are circular in nature. Whatever type of short story it is, the ending is built into the opening, and comes when the story circles back on itself, much like a snake biting its own tail.
Trouble ending a story usually means it didn't open properly.
The best way to know how to end a story is to know where to end it, and when to end it.
The simple version of this is to understand that a short story opens with a premise, a problem, and/or a question. It ends when the premise has been fulfilled, and when the problem has been solved, and/or the question answered.
The difficult part about the ending is that the reader should see it coming, but only after he's read it. It should catch the reader by surprise, but once surprised, the reader must be able to think back over the story, see the foreshadowing, and say, "Of course! How else could it end? Why didn't I see that coming?"
Two, somewhat extreme, examples of this are Gift of the Magi, and The Lottery.
These extremes are often called "twist" endings, but they still circle around and meet the beginning of the story, they're still the snake biting its own tail, and you should see both endings coming after you've read them.
As for punchlines, a strong last line is as important as a strong first line, but a punchline is rarely an actual ending. If it is, the story had better be short, and must be a setup for the punchline from word one. Any type of surprise ending is tricky, and if it isn't foreshadowed, it usually won't work.
First, a good ending is always inherent in the opening. A predictable or cliched ending just means you didn't take the hard left turn a good short story needs. It's good to see a predictable or cliched ending coming, and then turn it on its head. This is what separates a story that sells from one that doesn't.
Look for that predictable, cliched ending, and turn it upside down. Stand it on its head, but with foreshadowing a reader can see after the fact.
A surprise ending with no foreshadowing, that isn't inherent in the opening, isn't a surprise at all, it's a disappointment.