*Find tools you love, but don't limit yourself. Don't worry about "messing up" a pretty journal if that's what draws you. Don't feel silly if all you want is a composition book or a tiny spiral. Pick pens (pencils, markers, whatever) that write easily and that you enjoy using. I love Sakura Gelly Rolls. I also love fountain pens with colored inks, colorful ballpoints, scented markers...I choose my writing utensils based on my mood and on what the journal I'm using can handle (some have thin pages and can only take a ballpoint).
*Loosen up your idea of what a journal is. You don't have to write in it every day. You don't have to write down exactly what you did every day. You don't have to want to some divine, philosophical breakthrough. You don't have to entertain anyone but yourself. You don't have to fill every line. You don't have to use a certain kind of book or pen or color or organization. You don't even have to date everything. You don't have to name names. You don't have to limit yourself to happiness or sadness or deepest emotions. You don't have to write in cursive or print or in any consistent manner. You don't have to write at all if you'd rather make a collage, draw something, use stickers.... You don't have to write in paragraphs. Like my blog, my journal often contains lists. You don't have to write a certain amount. When I was a teenager, I often had entries that were thirty pages long. Now, I sometimes have entries that are one sentence, but I also have longer entries. It's inconsistent.
*When possible, be specific. When I read my old journals, it's not the soul-searching, deep exploration entries that fascinate me. It's the daily details, the descriptions, the meals, the clothes, the conversations and bits of dialogue, the shopping, the chores, the groceries, the momentary wishes, the bits that tell me who I was then.
*Find inspiration anywhere. If prompts help you, get a book or look them up online. I like SARK's Journal and Playbook and Judy Reeve's The Daily Writer. Recycle prompts. Just as you'll never read a book the same way twice, you'll never write on an idea in the same manner. I have a couple of notebooks just with ideas of writing topics--sometimes just a word or phrase. When you read a book or look at a magazine, have your journal and pause to write a sentence or two (or several pages; it doesn't matter) when something grabs you. Write after you watch a movie--not necessarily about the movie itself but about whatever the movie made you ponder. I have a scrapbook of images and words from magazines, and flipping through it can make me feel calm and help me write.
*Write on location. The most interesting way to write, which produces the most interesting entries to re-read, is to go somewhere. Bring your journal with you when you can. Write in the car (when someone else is driving) on your way somewhere. Write in waiting rooms, in libraries, at museums, at other people's houses, at parks, in gardens, on airplanes, in coffee shops, in restaurants. You'll find yourself pouring out expectations details, descriptions, observations, and passing dialogue. Writing on location has a magic and energy I can't explain.
*Make it special sometimes. Writing on location can do this. Journaling can be very restorative even if you only have half an hour. When I was working two jobs (waiting tables and retail--gah) and going to school full-time, I spent some time in the evening out on my patio, writing at my bistro table with a candle. Silky paper can do this. A new pen can do this. Writing in a bubble bath (though a little tricky) can do this. Make it a treat rather than a chore.
*Consume. In order to write, you need material. Watch movies, listen to music, visit people, go to shops, travel, read books, look at art books, have meaningful conversations. When you get that feeling of being full or itchy, write.
*Let topics merge. Write about work, your wishes, your family, your irritations, your gratitude, your aspirations, your creative work, your travels, your story ideas (or stories), your finances, your fears, your relationships...let it all come together just as it does in your daily life.
*Don't abandon it. You've probably read that one piece of cake isn't going to wreck your diet and doesn't mean you should give up. The same is true of journaling. Missing a day, week, month, or year is okay. Pick up the same book. I've done this after months without writing. One journal shifts completely as my life had done during that absence. It's interesting to look back on, and it keeps me from having stacks of notebooks with only a few pages filled.