A lot of amateur comedy writers fail not because their situations are not interesting enough, but because they are not happening to the right people. Take the following situation: Average, everyday dude just wants to be left alone on the subway but Andre the Giant sits next to him, very intimidating. Worse, Andre falls asleep and rests his head on the guy's shoulder. It's so heavy that the guy can't shake it away without distressing the sleeping giant.
There. Good situation, but not funny. Why? Cause it doesn't pass the Donald Duck test.
Now imagine it's Donald Duck on the subway minding his own business when Andre the Giant falls asleep on top of him. Bang! Instantly so much funnier.
Again: A substitute teacher enters the classroom to discover that the students have changed their name cards around and are planning on releasing a chicken into the room halfway through the lesson. If the substitute is your average, everyday Joe, then you don't have a comedy. But if he's Donald Duck? Yes!
If you can ever replace any of your characters with Donald Duck and have it instantly be funnier, then your characters do not have strong enough personalities. What makes a scene funny is not what happens, but whom it happens to.
This is why the Rob Schneider, Farrelly Brothers and Adam Sandler comedies failed. They evolved past the childish personas that made them famous and tried to play their characters too straight, and either they morphed into drama (The Wedding Singer, There's Something About Mary) or they simply flopped (Shallow Hal, The Animal, Mr. Deeds). Deuce Bigalow was always going to be terrible, but it definitely would have been improved as Donald Bigalow.
Now take a standard, average-good comedy: Planes Trains and Automobiles. Replace neurotic, status quo, salaryman Steve Martin with Donald Duck. Did it improve? No. Donald Duck just does not work with soft-hearted John Candy, because John Candy redeems the disaffected and we don't want to see Donald Duck redeemed. He's an archetype for avarice and rage. We want to see him punished. If someone is to love Donald we want to see him crushed in their embrace.
But it almost works, and it's almost better, but then again, PT&A is not the best comedy ever. Try it with Ferris Bueller's Day Off, Shaun of the Dead, Anchorman or Borat. Nope. No way. It just doesn't work. The characters are just too strong, too distinct. Donald Duck does not work. The only person we want to see get a day off is Ferris Bueller. The only kid we want to send back to the future is Marty McFly.
This is not to say the everyman is always a bad choice. Seth Rogan and Judd Apatow worked them to death, to great effect, and Monty Python comedy depended on the everyman. However, all of their works would have passed the Donald Duck test.
Next time you consider a comedic scene steer clear of the temptation to make it about some average, cool dude (i.e., your self-projection). Instead consider how much better it would be with Donald Duck and don't stop brainstorming until it wouldn't.