I'm just writing episode 6 of my sitcom and just wondering does anyone else write out the ideas for multiply episodes like I do for example I know this series I can have 8 episodes and chuck away the weakest two leaving me with nice solid 6. I just wondered how everyone else planned their series? Just write episodes as they comes or plan out the series then start writing
I tend only to write the pilots but plan the full series progression with threads developed for each main character in notes.
I never write a whole series as if the pilot fails to get interest then the other five have to be mothballed. I'd rather write six pilots than one complete series.
My writing partner and I started work at the same company three years ago and we got on like a house on fire straight away. Anyway, after some ridiculous conversation in the bar at lunchtime, he effectively bet me that I couldn't write a six-episode series synopsis in 15 minutes. Which I did. Three years later we've finally drafted all six episodes that are, amazingly, still based on that synopsis!
Though the 'Weak third episode' joke I made seems to have come true 
Dan
Apart from the usual scribble book I sometimes use a formset I knocked up many years ago. Not all the sheets get used as some ideas keel over quite quickly. Basically, if I think of a 'useful' character (doesn't matter what they might end up in) then I grab a Character Profiling form and jot stuff down. Might sound boring but I personally find it useful to have all this odd stuff in a file somewhere for later. Form Set comprises the following:
Series Header; Episode Header; Episode Planner; Situation Description; Interplay Description; Dialogue Drafting; Character Profiling; Location Profiling; Expansion Sheet (for overflows!) Series Glossary; Series Technical Data.
The Series Planner is Landscape format with 15 Columns for first fifteen minutes and below another 15 columns for the second 15 minutes. Each 15 minute chunk has 13 lines to write on. Six series planners laid out alongside each other represents a series. If I ended up with 8 then like Gavin I'd cherry-pick the best six.
Anyone interested in this approach can PM me and they will be rewarded (?) by an emailed MS Word file with all the forms and supporting notes. This is clearly not the only or even best way to do things but I find it helps.