I'd prefer the strategy employed by the writers of The Wire. Before writing the scripts, they break the entire season, episode by episode. A theme for that year is established at the outset, and once the season and episode outlines are completed (a collaborative process), the scripts go out to individual writers who complete their assignments. Then, the showrunner revises all of the scripts (sometimes making small changes, sometimes making big changes) by the other writers in order to make the season a dramatically coherent, thematically unified whole.
One of three other option I see are to approach writing like Babylon 5, and plan everything at the outset. I'm not much of a fan of this approach. It leaves the writers moving around characters like they are chess pieces instead of letting them grow naturally from what is on screen (the chemistry between Worf and Dax on Deep Space Nine, for example, was never planned but is incredibly apparent from their first scene together). It also leaves you beholden to real world changes that are unavoidable (mainly, cast departures), leaving the writers to play defense for their pre-planned structures. Instead of organically absorbing these changes into the series, the writing sticks closer to the original plan, leaving a stiffer, less interesting final product.
The next option is to make up everything as you go along, which is the strategy used by the writers of 24. Six (now a seventh) seasons of that series have proved this strategy mostly ineffective. The first season is somewhat ineligible for discussion, since the first 12 episodes were mapped out after the pilot, but the change in coherence in just one episode after that is intensely apparent. Seasons two and five managed to sustain themselves due to writing that was as clever as it was lucky, but the other seasons are far less coherent--the third, fourth, and sixth years all have Frankenstein-like narrative structures, with abrupt, arbitrary, and incoherent plot twists that do their best to keep ideas that have petered out after six or twelve episodes on life support for a full, 24 hour season.
The last option, which some will not differentiate from the previous method (but I will) is the one utilized by both Farscape and the 2003-2009 version of Battlestar Galactica. This is an approach that is not entirely without a plan, but does not have a precise structure. Instead, the writers work with a few story points they know they want to hit in a particular season, but that's it. The rest grows organically from the writing process, sometimes for better, and sometimes for worse. I can understand why some writing staffs use this strategy--I doubt writers outside of a few cable series like The Wire or Dexter have a lot of time (and money) alloted to methodically break scripts before filming begins. And, usually, they have more episodes to produce in a season (perhaps twice as many), further accelerating the schedule.
24 is much better if you just think of it as stealth sci-fi.
