I used to be afraid to write in a blank book. I felt that, if I started a story in one, I had to finish the story there, or else I had "wasted" it. I loved having blank books, though, and so have amassed quite a collection.
Junior or Senior year of college, I got over it. Maybe it was because of the creative writing classes I was eventually able to take on the side (my degree is in Psychology). Suddenly, I was able to write a name, or a blurb, or the beginning of a story, and if that was all I was going to write on that topic, I could draw a line or turn the page and write something else. Easy peasy.
It was freeing, really, to not have to worry about keeping a book "nice". I think I had probably read too many books in which the heroine had journals or letters that she kept for posterity, for Those Who Came After to read. Between computers and my handwriting, I don't really need to worry about that. Frequently, I'll write the opening of a story in a notebook (I keep any number of them in my purse and at work) and then finish the rest of it on the computer.
Other times, I'll have an idea, but not really the time to get out the pen and paper. In those instances, I'll whip out the cell phone and email or text myself the phrase that's come to mind. That's how I preserved my idea for this year's National Novel Writing Month; I was almost asleep, lights off, pen and paper distant, and thinking about the stories that we tell. I didn't want to forget my notions, and so I grabbed my phone and emailed myself the words "Family Apocrypha". And then went to sleep and forgot. Imagine my surprise, the next morning, when I opened my Gmail! But I remembered the idea.
Good ideas or bad, if you don't remember them, you aren't able to judge. Write it down.