I've experimented with something recently that I thought might be worth sharing. Some of you might have noticed, if you look around at various CFPs -- both for conferences and special journal issues/book volumes -- that some CFPs ask only for a paper abstract. It is usually implicit in these CFPs that you are not expected to actually have a paper ready-to-hand. Usually, they say something like, "full paper drafts for accepted abstracts will be due on date X if your abstract is accepted."
Over the past year or so, I've tried to take advantage of these opportunities. When I come across one of these CFPs and think I may have an idea worth pursuing on the topic, even if I don't have a paper fully worked out yet, I'll quickly write up an abstract and submit it. I did one just this afternoon, in fact. Interestingly, I've found that doing this has several potential benefits. First, and most obviously, if your abstract is accepted, you have a chance to present or even publish a paper!
But this isn't the only -- or, in my experience, even the best -- potential benefit. Another benefit is that it gives you some primer and incentive to think about things and develop paper topics on issues that you might have otherwise never engaged with. For instance, before I came across this CFP today, it hadn't occurred to me at all to write a paper on this topic -- or that I would have anything to say about it. However, after the CFP prodded me to think about it a bit, I came to the sudden realization that, my goodness, research I have already published on has surprisingly interesting implications in this area. In short, I discovered that I may have something interesting and potentially publishable to say on the topic, and I probably would never have known it if I hadn't come across the CFP and gave writing up an abstract a shot.
I don't know if this will work for everyone, but if you do have trouble coming up with good paper topics or just want to push yourself to come up with more topics than you presently do, I suggest giving it a try! It's actually a lot of fun, and even in cases where I haven't been able to come up with anything good, the exercise itself (which needn't take all that long to do, in any given case) can be instructive (after all, learning that an idea that's been rattling around in the back of your head isn't good is a victory of sorts too!).