I play music reminding me of the stories I'm writing, even when I'm doing something else. These are also the soundtracks I use while writing, so hearing the songs again stirs some creative thoughts up.
It's great to be able to just write for the heck of it, without worrying about what you'll be able to use later. So if you haven't written in a while, starting off with some low-pressure writing might be exactly what you need to get going.
Starting off just chatting with characters can give amazing inspiration in surprising directions. This also works extremely well when your story's starting to feel dull.
Reread what you've written.
Not recommended if you're an over-editor, but sometimes, all you need to get back into writing gear is to relive the awesomeness you've penned down before. Remember to look on the bright side and to ignore the nitpicky issues.
Start off by writing something totally random.
The hardest part about writing after a long time away is starting. Writing something random tends to open up the writing channels, letting you think what you actually write down. I once broke a six month writer's block by opening a chapter with: "The gunk stuck to his mouth like peanut butter. He hated peanut butter." Funnily enough, those sentences ended up in the final cut of The Vanished Knight.
As you might see, all of those suggestions have to do with chilling out before you start. Forcing yourself works too, but it's much easier to want to write. It's always trickier to start when you're panicking about how to do it. And the how's and why's almost always come when you're not looking for them.
I hope this helps!
Anyone else have pointers for getting back into writing routine?