If your idea is really good, trust me, it'll come back to you. It'll drag you out of bed and say, "Hey, I'm still here! Get out and let's create something beautiful!" So, if you decide to quit, well, maybe the idea wasn't that good.
But I understand, sometimes inspiration slips away. So here are some motivation boosts:
Choose a font that you can't stop staring at.
Not sure if anyone have figured out this yet, but having a sexy, nice-looking font can make you write for hours. Here's mine (Gentium Book Basic):
Remember what excited you about your idea
This is similar to a relationship: you have fights, arguments, moments of hatred, but you can always smile again by remembering the first date. Do the same with your story.
Remove the bits that make you bored/unmotivated
If you're feeling bored or unmotivated maybe it's because the scene/chapter you're writing is not as exciting as your original idea. Therefore delete it.
Think about how your story will change the world
One of the greatest joy in life is to help others and contribute to society. Think about how your story will accomplish that.
Don't stick to only one story if you don't feel like it
You want to develop a story that had been in the back of your mind? Do it. You can always come back to the previous one. I won't run away. (Unless someone steal it, but that's another story).
Coffee is a stimulant (to become imperative). Alcohol is an uninhibitor (to let unfiltered ideas out). I know what they say: you don't need drugs/chemicals to be creative. But let's be frank. They help. It's no coincidence that Hemingway, Salinger, and Bukowski were drunkards.
And finally remember this: you're the only one who can finish your story. No one else can. So you have the responsibility to bring that story to life, show it to the world.
(By the way, Henry Taylor is right: deadlines help.)