Sometimes I have to force myself to start. But once I do, writing it becomes pretty simple.
Having said that though, if you really don't feel in the mood to write, don't. On those days when the last thing you want to do is write, just take a load off. Because you force yourself too hard, you won't think about what you're writing, and whatever you write is going to be baaad. Trust me. Been there, done that. You shoot yourself in the foot when you have to edit out all your screw-ups.