I am sure that everyone in a creative role has experienced that situation, and probably more than once:
You are working on something (a report, a competition, an exam, a novel…) and a very strong deadline looms. You don’t have time for distractions, you can’t fool around. You need to keep focused.
And of course, this is precisely when dozens of ideas pop up in your mind (short story ideas, for instance), all absolutely amazing…
…and none related to the task at hand. In fact, if you follow them, you could very well a) lose track of what you are currently doing, and b) not have enough time to complete what you are doing.
But on the other hand, if you don’t work on these new ideas right now, you might lose them forever…
What can you do?
The answer is: keep track of the ideas. Take two or five or ten minutes — no longer —, write them down, and file them. For instance, if you are a writer, keep an idea box / file / folder, and every few weeks or months, have a look inside.
Try to be as exhaustive as you can — you won’t go very far, two weeks from now, when you reopen the file to see:
“Boy finds artifact, doesn’t know but of great power. Goes on quest, is almost destroyed but ultimately saved thanks to friends. Great idea!!!!”
But at the same time, don’t try to be too exhaustive now either. You want to capture the idea, the mood, the feeling — not kill it. Straight off, it’s very unlikely that your idea will already be perfectly refined and effective, so don’t bridle it. Let it run its course. In a couple of weeks, your subconscious might come with some improvements, so don’t close the door for them right now.
And after that’s done, come back to your work and finish it.
First — it probably depends on people, but I am not a big believer in multi-tasking. Unless you are really stuck and need to change your perspective, it is more efficient to work on one project at a time. As a writer, I think of it this way: building a scenario is like weaving something. You have myriad strands, hundreds of knots, and you try to use them all to weave something good. Efficient tools (e.g. spreadsheets, scenario-building programs, etc.) will help you keep track of all the threads, but they cannot replace your working memory. When weaving, you can leave your work on loom for a night or for a few days, but if you go for several weeks, you might not find all the yarn in the position you left them in. Similarly with scenarios: if you have forgotten the existence of this character or that theme, it will take you some time before you remember to use it right at the spot where it makes sense.
The second reason is relative perception. Right now, all these new ideas look very good to you — but it could very well be just a relative call, compared to the thing you have been doing for the past day, week or month. At the moment, you are probably bored with what you are doing, and as a consequence, everything else looks marvelous, including taking out the trash and cleaning up after your neighbors’ poodle.
Recover your sense of perspective.
Take a break — 5 minutes, 5 hours, whatever you need and can — and reassess. Is this really the greatest short story idea of all time? Or is it more like Yuki, the neighbors’ poodle? Or (more probably) is it good — but not as good as the novel you are currently preparing?
This is why you need an ideas box — and a couple of weeks to let the ideas rest, and regain their true proportion.
And in the meantime, write.