Brevity really is the soul of wit. A too-long joke isn't just unfunny. It's also annoying. Our example joke is stripped down to the bare essentials. There is absolutely no unnecessary information. What movie is showing? Is the usher sweeping up popcorn? What color are the seats? Who cares? You've likely heard someone tell a joke and insist on including details like these. It wasn't funny, was it? Information like this can really damage a joke. Too many details can distract the audience - or worse, the joke-teller - from the important pieces of the story.
Remember, the shorter your jokes, the more time you'll have to tell other jokes.
A Few More Tips
Never laugh at your own joke, especially while you're telling it. Let the audience do the laughing for you. Also, never, ever say: "This is a good one." If a joke is funny, it speaks for itself.
Study the experts. There's a reason famous comics are funny. It's because they're good at what they do.
If you think you are funny, you probably aren't. Funny people know they're funny, and never worry about it.
If you're trying to hard, it will show. Sweating and telling jokes do not go together. Nothing makes an audience more uncomfortable, and less inclined to laugh, than someone who's obviously having difficulty. Confidence is key. That's why practicing is important. Keep in mind, though, that over-rehearsing can kill the necessary spontaneity of a joke.