Whether you are a designer, writer, new business director....whatever. You need to write your ideas down WHEN THEY ARE FOUND. It's fresh and you will you have a clearer understanding of what made the idea good if you write it down at that moment. And it's best to write out a full thought. It gives that idea a better chance to live. For whatever reason I tend to get ideas when I am driving, walking, jogging, biking, etc. Moving has always helped me clear my mind and think. But of course this also means I am not sitting at my computer where I keep all of my full thought plans and ideas to execute.
So I tend to have notebooks stashed ALL over the place and with me at all times.
Rather than trying to use one notebook at a time and worry about where it is...I keep notebooks in my car, in my laptop bag, in my living room, in my wife's car, everywhere I hangout. Honestly if I go for a walk or even a jog I have a phobia of not carrying something that I can write on in case I find an idea. Driving and running tend to be where I most often have this problem.
I started buying Moleskine notebooks a while back and I can't imagine using anything else. Specifically the Moleskine Cashiers Notebooks....kraft brown. I really like the simple design and small pocket size. I like that they don't have a ton of pages in each book so if I use some of it and it gets trashed it's no big deal. I buy the 3 packs and keep our cars stocked with them. They don't look trashy in the living room or kitchen and I can easily keep these in my back pocket when I am walking to lunch downtown or even when I am jogging.