Just thinking with critiques current purple patch how about accumalating our wisdom and advice gathered from it. The how not to advice based on what you wrote or read that made you think, man (or lady) don't write that!
1 Joke grasping.
When you go for every single possible gag. When you're doing well the jokes you miss are as important as the ones you keep. If you're sitcom has 2 characters stop just to exchange witty lines for a page. Or you're hilariously stupid groundskeeper stops to real off some Wildean wit. Then you're breaking the spell and those jokes will hold you back.
2 Over explanation.
My pet hate. Humour is very much about surprise and intellectual leaps. So every time you painstakingly set up a gag, you're draining all the humour and surprise out of it.
Starting a sketch
"Hello I am professor silly trousers and today I will be hilariously explaining gayness to penguins."
Will never be funny unless his speach turns out to be dead serious.
3 Running past the punchline.
The punchline is where the joke ends. Anything extra is just a bit confusing. And akin to running upto a heavy weight boxer in a bar punching him in the cock. And then hanging about.