Keeping a journal is a great way to write regularly and create potential aspects of future works. A journal can be a fancy leatherbound type, but it's just as effective in a cheap spiral notebook. The important thing is that it doesn't simply sit on your bedside table or get covered in candybar crumbs at the bottom of your backpack--it should be filled with ink. And writing isn't all you have to do. Many creative people find it just as inspiring for future reference to draw or paste pictures in the pages. I find that magazines are great for finding a character. They're easy to describe since you already know exactly what they look like, plus you can refer back to them later and your memory of the color of her hair or the slant of his smile is immediately sharpened.
The only rule about your journal is to make it truly yours.
When I taught creative writing I often gave daily journal assignments. Do what you like, but I'll offer you the same guidelines I gave my students:
For the fifteen minutes or so that we've set aside for journaling, just write. Take my journal topic as a jumping off point and go from there. It can be true or totally fictionalized. If your story takes the shape of a play, that's fine. If it is a character sketch, great. If it's simply a description of a setting, perfect. Listen to your creative voice.
Your journal is your gym. It's where you exercise. Most gyms are stinky places covered in sweat and people pushing themselves to become their physical best. Journaling doesn't have to be pretty. Just push yourself to go beyond your comfort zone.
You'll be surprised at what you learn you can do.
In fact, the prologue of my novel Glamour was a journal entry. My character Kat just arrived in my classroom as I journaled with my students. True, she was inspired by some boys (that I usually adored) who were excited about something that happened at lunch and insited on whispering, gigging and disturbing the others who wanted to journal. I'm thankful to them, though, because when we all managed to settle down, I had the first line of my novel: She liked to keep their tongues in her pocket.
Try out the journal topic for yourself and see what you come up with.
Journal topic: What do you have in your pocket?