I find that the hard part comes from not knowing what to do next. For me I have a notebook that I call my planning notebook. I write in the notebook the outline of what I'm trying to accomplish -
Who it's for
What it should do
When I think it'll be done
Where it needs to happen
Why I'm doing it
How much it will cost
This frames the project for me. Then after that I come up with what I think the next thing I need to accomplish on the project is and write it down. Later when I site down at my computer, I have no questions about what to do next and so I can hit the zone really fast.