Came in dead last in the LEVERAGE Oscar pool, so you may wish to take any industry advice I give you with a grain of salt. (What, we're going to let Hal Holbrook die without his Oscar? You heartless bastards.)
I have, in front of me, 113 scripts to read. Well, virtually in front of me. We're using Box.net to upload the samples as pdf's, so the responsible parties can read them and funnel feedback to our redoubtable Writer's Assistant, who then collates the information on a cunning spreadsheet and schedules follow-up meetings accordingly. A good chunk of these folk are for mid-level and up, meaning they're starting with better credits than you. Assuming, of course, you got an agent to submit you into the pile. All this for 12 episodes on a cable network. Times have changed.
So how do you claw your way to, if not the top, the upper middle of this slab? I'm beginning to evolve a new bit of advice now that I'm actually on the receiving end. Write a spec show that's on the air for agenting, and a pilot for staffing.
Spec shows, if they're written properly, are painfully particular to the show in question. Your entire job when writing a spec episode is to show that you can learn and adapt to a show's specific rhythms, making you a valuable addition to any staff. (You never spec the show you're trying to get hired on. But you know that.)
The problem is, it's not ten years ago, and there's not a damn bit of guarantee that you're speccing a show the show-runner watches. Ten years ago, if you specced one of the top ten or fifteen shows, you were speccing based on some sort of roughly shared cultural canvas. Now -- once you drop out of the top ten shows, the next fifty or so all smear out to roughly a ten share. One in ten people watch the show you're speccing, odds are, and showrunners are often even more eclectic in their television tastes.
It doesn't help that many of the top one hours in particular, the CSI's etc, are all enemies of character development. They're elegant story machines, to be true, but I usually wind up pulling a writer in when they make me enjoy one reading of the procedurals despite myself, by slipping in a nice bit of character conflict under the Clue Machine.
These skills do convince fine Agenting Humans that you are employable, and are so valuable skills. But as soon as you have an agent and can reasonably write a pilot without making a total ass of yourself -- do so. This not only displays for some poor script-buried showrunner your raw tools, what you can do without both the constraints and bracing beams of an established show, but it's a cleaner reading experience. A whole meal, if you will. I now know what kind of stories you're going to pitch, what character beats you find amusing, what kind of dialogue you naturally gravitate to, what relationship dynamics you enjoy exploring ... this also helps if you're coming off a show, and you want to work on something a bit different. I never pass on a writer based on credits, but some people certainly move scripts up or down in their reading order depending on them.
Also, two samples is kind of the minimum cover charge for specs, these days. You want to show a bit of range, and just in case the pinheaded showrunner doesn't respond to your first opus, hit them with the change-up.
Oh, and one more note, for established writers -- it wouldn't kill you, if your agent's submitting a produced episode, to slide a character/plot summary page in there. Takes thirty seconds.