I think I must have first started keeping ideas books/ an inventors journal about 10 years ago when I read book about inventing that advised you to chronicle all the ideas you had along with a date.
A video explaining the importance of an inventors journal
Now I have to admit I am not quite that organized and sometimes forget to put the date which is really stupid of me (must do better), but at least I have a record of all my ideas. If you think about it, imagine how many ideas you have had over the years and how many are now long forgotten. What I have found is that every now and again I will flick back through some of those ideas and some may even be combined with new ones I have as I did with a recent character design. It’s quite weird when that happens, you have an idea for something, in my case a kids character, but there is something missing, open one of your ideas books and there’s your answer. Prior to keeping ideas books I just used to scribble on bits of layout paper which now lost forever as I never have been particularly tidy.
Write down any idea you have however daft or insignificant it might be even if it’s just one line of text to jog your memory (remember to include a date and get someone impartial to sign it – unlike I do!). Don’t censor yourself and your ideas, you can question it’s merits later on, for now just write it down. I like to add drawings to mine, well drawings is perhaps stretching it a bit in some cases, more of a doodle. Even though I can draw reasonably well your ideas book is not about drawing skill it’s about capturing the essence of an idea so that YOU know what it’s is. No one else has to see your idea books so stick figures will do just fine as long as you understand it.
I use hardback sketchbooks so they won’t get to tatty and will keep for many years. Don’t get a really expensive sketchbook or you will do the daft thing, many of us creatives do where you don’t want to spoil it by writing in it (well that’s what I am like any way)!
Read more on tips to keeping a good inventors journal at http://inventors.about.com/cs/logbook/ht/Log_book.htm