If you're a designer I highly recommend keeping your ideas in a sketchbook that you keep with you at all times. You never know when inspiration will strike. As you can tell from the page out of my sketchbook here, you don't need drawing skills, either :)
One thing that's fun about it is seeing the difference between the sketch and the finished item; often the design doesn't work out quite the way you wanted it to, the material has a bit of a mind of its own, and you just want to refine it a little bit. You can also go back through the pages of your sketchbook when you're stuck for a little inspiration.
Three of the sketches on this page have become actual items, pictured in the order described. The first became "Sylvan" and since I didn't have a donut, it got a charm, and some chains, and a little bit of a change to the weave. Actually the sketch looks more like "Aurora," (not pictured) which I did later. The second picture became "Matsuri", although I have another sketch that looks more like it, and I may do something like what I have pictured there, as well. The third is a work in progress, and the last is "Dervish", which came out pretty much as is. The little picture to the right on the bottom didn't work out at all; I didn't like the weave.
These days I tend to keep my sketchbook open in front of me while I'm doing my medical transcription work. Anything repetitive and mindless is good for third-brain brainstorming, and
transcription qualifies once you get going. While I'm not recommending sketching while driving, that's a good activity for brainstorming too since if you know where you're going your brain goes into automatic mode and can focus on other (right-brained) things while the left brain takes care of the repetitive task. Working with a sketchbook in front of me isn't all that conducive to getting lots of work done, but it does seem to be when I do my best thinking.
(someone pointed out to me that it has the added advantage of proving that your designs are your own, if anyone every questions that for any reason)
And if you're obsessing about design like I tend to, you'll be thinking about it all the time anyway, and when inspiration strikes the sketchbook will be there. When you see a sea and sky picture in a magazine you can clip and paste. When you see a pattern in the sky, in a spiderweb, on... heck, even the jewelry someone's wearing, the sketchbook is there to record it all so that it can come to fruition later. You might even keep a little journal recording what you were doing when you thought of the idea, or other odd little details about it.
If you don't use one, definitely try it. You might find inspiration striking at more odd moments than it used to, because you can take it to the bench when you have the time to create.