"It is necessary to write, if the days are not to slip emptily by. How else, indeed, to clap the net over the butterfly of the moment? For the moment passes, it is forgotten; the mood is gone; life itself is gone. That is where the writer scores over his fellows: he catches the changes of his mind on the hop." ~Vita Sackville-West
It's a rare thing that you sit down to write and some of your best ideas actually come while your writing. If you're like me an idea will come to you out of the blue when your driving the car, making dinner, working-out, or maybe even watching Television, and if you are also like me and don't write that idea down right away it will disappear into the haze of your memory. It is important to catch those thoughts and place them somewhere so that they don't drift away to be lost forever, because chances are they won't come back. I can't tell you how many great story ideas or ideas period that became lost to my forgetful brain. So what is the cure to this dilemma?
I wish I could say that there actually was a cure to the haze of memory, but I do know there is a way to get around it. Just write your ideas down as soon as they come to you. The best way to do this is to keep a small notebook or journal with you at all times that you can get to without too much hassle. You don't want to go looking for your notebook and find that in the process of looking for it that you completely forgot what you wanted to write down in the first place. Keeping a notebook near you might be a little awkward at first, but you'd be surprised how quickly you get use to it and rely on it. If you do happen to find yourself without your notebook, then find a scrap of paper to write on.
Whether you use a notebook or paper scraps just get those fleeting thoughts down in writing so they don't vanish completely. You'll be thankful for the ideas later on and you'll find that the habit of writing, even if a few short sentences or words will actually compel you to want to write even more. Catch the butterfly of the moment.