A notebook of one's own
I simply can't think of an artist who haven't carried a notebook, taking it to wherever he/she goes, as an inseparable friend. Better, as part of him/herself. It doesn't need to be a fancy Moleskine; as long as there are blank pages ready to be filled in it, it will serve just right. And sometimes they just seem to breed while we're not looking - take your first step and buy a single book to write everyday, to develop this habit, and suddenly you'll have about ten, twenty of them!
It really doesn't matter if it looks messy for other people, but it is very important that you can understand your own junkyard. If you don't, find a way to better organize your note-taking process. You can keep separate notebooks for separate themes or purposes, for example - a daily diary, a journal for dreams, another one for interesting news, another one for loose ideas and brainstorming, another one for first drafts, and so forth.
It is said that you can tell what a person is like by analysing certain elements such as handwriting, layout organization, and graphic characteristics in general. But, honestly, if you take a look at my notebooks you will probably think that I have multiple personalities... oh, well, I guess I do. As a freelance writer and illustrator, I usually keep a notebook and a scrapbook in my backpack, and I keep separated ones for academic and research purposes (those are much more neat and well organized). I keep no daily journal though.
A way of overcoming writer's block
Because there are no rules nor boundaries to what you can put in your notebook, you don't have to worry about the outcome - just draft, scratch, scribble, glue pictures, make collages, clip pieces of texts, and that will help you to come up with new ideas.
Author Ray Bradbury, in his book "Zen in the Art of Writing: Essays on Creativity", tells he used to make lists of nouns or quick expressions. He would sometimes go back to those lists and just write about one specific item; those worked as memory triggers.
In fact, keeping notebooks can help you not only to trigger your memory, but also to know more about yourself and to better explore your thoughts - sometimes even long lost thoughts you had forgotten about. At times, reading a single word listed by you months ago will be enough to awaken a full short story or poem - let's be honest though, at other times it won't do anything for you.
If you're more of a visual person, you can look for pictures in magazines that catch your attention. Glue them on a blank page and see what you can write about what you are looking at. Or even about the situation of looking at a page with a figure and not knowing what to say about it. Write with different pen colours, maybe according to your mood, or according to the subject. Write about how you feel when you're writing. Make a list of your favourite words, separating them by sound, or by meaning. If you are extremely organized with your things, try to do a very messy brainstorm, by closing your eyes while writing or drawing. Always challenge yourself to get out of your routine; when it comes to a notebook with blank pages, the only limit is its size!
A remedy for anxiety
When we are anxious, our writing tends to reflect it. As a result, we'll have confusing texts, conveying blurring ideas. Notebooks are a good tool, serving as a release medium to this anxiety. Once all thoughts are on paper, we then become able to link them as we please and to try different connections. Only after this phase we can really write clearer paragraphs, since we no longer have to worry about remembering every running idea. At this point, planning is much easier and you will probably be less stressed.
You might also want to keep a separated notebook for quotes from other authors and one for your own quotes - so you can easily find those good phrases or paragraphs you didn't really know where to shove when you came up with, but can eventually be useful. Again, use colorful pens and sticky notes to somehow organize themes or to call your own attention to specific pieces. If you are feeling lost or blocked, consult those phrases and they may offer you a glimpse of an idea.
Carrying a notebook is also an effective way of not getting bored during those annoying situations when you are forced to wait - in a doctor's waiting room, at a bus stop, at the airport, during a family reunion while your uncles are arguing and your cousins screaming...
By the way, a simple notebook is more portable and easier to "load" than a laptop. And there are significantly less chances to loose your "data".
An example of how to organize a notebook
Keep track of your evolution
I still have notebooks from my childhood - since kindergarten. I can spend hours and hours reading them, looking at drawings and silly poems... There's such a great sense of nostalgia surrounding our old "art"! Those also can be sources of ideas for new creation.
Depending on the number of pages, a notebook will last awhile before you put it in a shelf or inside a box. Either way, you can always easily take a look at your past brainstorms and drafts, having the opportunity to assess your evolution, to see if you are satisfied with your own progress. Try to date each page so you can check your daily productivity - if you notice you've had a very productive monday and a less productive tuesday, for instance, maybe you won't be so upset because one day's amount of writing can compensate the other's. Or a beautifully written page can be an additional motivation for you to carry on. And if you eventually become famous, those will make incredible material for a biography!