Always Be Writing. Pick a time of day and just write for 10-15 minutes. Even if it's just a grocery list, a random bunch of words, a letter to yourself, whatever... just the act of writing is sometimes enough to "warm up" and get the thought process going.
Use a Pen. For myself, anyway, the physical act of writing puts my brain into a different area than writing words on a screen. It 'feels' different, like I'm doing something new instead of being on a computer and my brain pays attention.
Make Mistakes. Don't assume your opus will magically create itself the first time you come up with what you think is a great idea. Oftentimes, once you realize where your writing is "going wrong", you'll be able to steer your narrative in a new direction, or new avenues will open up to you. It will be frustrating, but rewarding in the long run.
Don't feel like writing? Get a digital recorder and just go for a walk. Sometimes, unplugging and just getting the body moving puts me into a zen-like state. Without my regular distractions, I'm forced to invent new ways to occupy my time which often unlocks some creative neurons. I find I'm at my most creative when doing dishes, in the tub, or even on a bus... anywhere that interrupts my daily routine.
Do New Things. We often write from our own experiences. If you're not out there doing things outside your own writing desk, you're not getting the full picture of everything you can offer to your readers. Live a little! Meet some new people, go to a new restaurant, or a museum, or take a weekend to yourself and stay in a new town/city. Things you do and learn and feel will be reflected into your future characters.