Screenwriting is a business. When you're a writer, you (your work) are your product. Your ideas and your talent are the good you sell, and your ability to deliver on time and be a pleasure to work with helps put you above the rest of the competition. And, like in any other business, in order to stay afloat in it - or in cases like mine, where the writer has yet to make a sale, but is trying to wiggle into the industry - you must know both the market value of the product and the market trends.
I've talked a lot in the past about the importance of knowing the market. Sites like Done Deal Pro and Deadline Hollywood help track sales and industry happenings. Track this information closely. You can tell what the value of your idea might be based on whether or not something similar (i.e. teenage vampires) has sold recently, and what it went for. If twelve teenage vampire scripts sold last month, there's probably a good chance that by the time your idea hits the right agent or producer's desk, the trend will have passed, and the next hottest tween 'tainment craze will have begun. On the other hand, if space seems to be the next frontier for Hollywood, and your agent just happened to have received the next Star Wars from you, you could be in a good spot. Similarly, if a couple big action movies just sold, and you have a tentpole action out to representation or studios, you can guesstimate what its worth by what the others went for. (Of course, your representatives are the ones who will negotiate the terms of the sale; their job is to know that X scripts are going for Y money these days. A writer in the know, though, is always a smart businessperson.)
The downside to all this information can be equally as obvious. Research done at Box Office Mojo and via the trades can and often does shed some light on the climate for certain pictures at the moment. For example, the past week, I've been developing a Middle Ages idea. Over Thanksgiving, I watched the new Robin Hood movie. It wasn't very good. More than that, it cost a lot of money, and it was not considered a huge box office or critical success. Failing on one of those two points might not be so bad, if the other lives up to hope. For both to fall short, though, can be bad news - not only for the companies and talent involved, but for the genre itself. I had a hunch that Robin Hood might have made Middle Ages movies a bit risky now, so I called my manager last week just to follow through. Sure enough, he was keen on everything about the idea, except for the fact that it was set when and where it was. Audiences did not receive the last Medieval movie well, so all future projects are (almost immediately) dubbed too risky. Especially when written by still-unknown writers.
On the other hand (and back to the positive), tracking the trend like that allowed me to get a jump on debating the viability of the project, before getting too far into the script. Imagine if, three months from now, I sent my manager a completed draft of a script, just to hear, "I like it, but we shouldn't do the Middle Ages now." That would be bu hao ("no good"). So, in addition to all the writing you're doing, if you don't do so yet, try to follow the market, as well. It can give you a much needed leg up in this difficult industry.