Half of your success as a writer will be determined by the strength of the screenplay you are trying to sell. The other half is determined by your ability to pitch and sell your next projects.
No agent/producer/lawyer/manager/studio wants a one-off writer. All those entities, whether they outright say it or not, are in the franchise business, and you, my friend, as a writer are a franchise. To be the most valuable commodity you can be, to get all of those people and buyers and businesses behind you, you need to give them a long-term reason. That reason, naturally, is a series of ideas you can continue to produce for them.
When I took my meeting at UTA (can it really almost have been three years ago already?), my manager worked with me to prepare three pitches for additional specs before I even got in the room with the agent. Sure, I had a strong script that we all thought would be a relatively easy sell (it wasn't - so sad), but that wasn't enough. No matter how great that one project was, my agent's time was more valuable than a single sale. I needed to show him that I'd be around working and earning for years to come. But, let's step back a moment - before UTA was even on the horizon for me, I was settling into a partnership with a pair of producers and a new (to me) manager. One of the very first things my manager asked for was another script of mine. He, too, was testing the waters of my writing; he was relieved when I said that the somewhat inferior script I gave him as a followup (the only other viable one I had on hand at the time) was an earlier script. It wasn't as strong, but I was younger when I wrote it. Similarly, if you recall, when I met with a publishing friend about my children's book, one of the first things she advised me to do was come up with other book ideas I could mention in meetings with potential agents.
Clearly, having additional ideas is valuable - and it is best to keep them in the same genre as the project that you're gaining attention with, since that is where your strengths are most immediately visible to people who have only minimal, but favorable, exposure to your writing. Sometimes, though, it can be hard to focus on what's next when you're in the middle of a large project, or even when you're coming down onto the tail end of it. I recently got the (hopefully) last draft of the sci-fi collaboration to my partner. He liked it, and aside from a few notes we'll discuss in the coming days, didn't see the need for many more edits. If that's in fact the case, then we'll give it to our producer and representatives next. Barring glaring errors or holes that they might discover, we'll be fast approaching the "try to sell it" stage, which means looping my agent in. Yes, that same guy at UTA and, no, he's not aware I'm working on this yet. There's no reason for him to be until there's a product, since I haven't really been on his radar since the post-Apocalyptic spec failed to sell. But, the moment I call or email him to present him with a spec I wrote for a know director, all that will change. At least, it should. And I will be shooting myself in the foot if I don't have a laundry list of the next specs I want to write ready to give him. So it is time to put on my thinking cap, dust off the old ideas, and determine what projects I want to work on next.