I always use the following rules in my episodes...
(Please note I write in Film Script format of 1 page per minute)

Hook - Page 1
Turning point - Page 7
Climax - Page 15
(accomodate for an ad-break)
Hook - Page 16
Turning point - Page 21
Climax - Page 27
Resolution - Page 29
Hook - Page 30

Look at from a psychological perspective. A successful episode is one that takes the audience on a journey for half an hour or so and leaves them wanting more. We're not just zombies that come back same time, same place, next week. We can download whatever we want these days. But to keep people keen, give them a reason to start looking for the next episode as soon as they've just seen the last one.

The first (or pilot) episode, in my opinion is always the worst. Did you ever notice how in American pilots the actors look a lot different, or even change come the second episode? Did you ever see the Simpsons where Homer saw a pilot with a character that had his name, and everyone thought he was cool. Then the next episode they changed the character to being a bumbling fool, so he changed his name to Max Power?

Look at pilots and tell me they are the best episodes of the series. I'm a big fan of Spaced, but are you gonna tell me that the first episode was the best? Sorry Mr Pegg and Ms Stevenson.

A pilot is simply the first episode. Here's a rule that I follow. Make every episode the best you can, and try and make them howl for an encore.

I think, being realistic, you have to write your pilot bearing in mind that the person reading it, however well-intentioned, will spend a great deal of time plowing through spec scripts.

I agree you've got to hook them with a really strong early gag, just to show you can actually write something funny. This will encourage them to stay with you over (at the very least) the first ten pages.

Your characters and situation should be original and defined within the first ten pages. You have to orientate your audience. Also, remember, whatever your personal vision for "breaking the sitcom conventions", you have to have a plot, which has to be set-up early and engage your reader to at have at least a passing interest in where your characters are going in this story.

Write tight, funny scenes. Don't ramble off your plot path in a pilot too much. Focus on your defining your characters and structuring thirty minutes of plot-led comedy drama.

THEN...

Put it in a drawer/computer folder for a couple of days and think about what you've written. Do you really think this idea is worth carrying on with?...

Well, of course you probably do...

Re-read your initial script... 'This is shit!' They always are. Now you really have to start writing.

It's not funny! Never mind at that at the moment. Go back and re-work your scene structure, plot points and charcterisations, even if it seems to remove jokes and turn your script into a very dry afternoon Radio 4 play.

Put it away again and go and get drunk/laid/stoned/religious for a few days.

Think about your characters and your plot. You'll be suprised how you start thinking of the funny things your characters say. Mentally log these, but don't write them down as one-liners or gags...

Right, now that you've sobered-up and become an agnostic, go back to your script and try and make it funny. You keep your characterisation, plot-points and scene structure but replace the dialogue with something that's funnier.

Then all you have to do is repeat this process over and over and over again, until there's not one word you feel you can do better with.

By doing all this you may end up with a script that you might actually have the guts to show to a sympathetic friend or family memeber. You may have to start over again.

This is only one of many ways to do it. By your third or fourth pilot script you'll discover your own rules and regulations to how best write comedy.

This may all sound very cynical, but hey, you're a comedy writer, you're naturally cynical. And for all those who say that they want to write a ground-breaking new kind of sitcom, I say 'Great, so do I'. But think of all your favourite alternative and mainstream sitcoms. They were all written by people who worked very, very hard on the structure, rules and plot of their comedy, however unusual their vision was.

On the other hand, what do I know?

Good luck with your writing.

Excellent advice.

That seems a bit like hard work.

Tim Walker, spot on. Writing a draft is the easy part. Rewriting is the art.

Pin Mr Walker's advice, someone! The man is spot on and succint as well. No wonder he is making progress (and good luck with it, too).