My standard reply:
http://www.obrienscafe.com/adventuredesigner.html
A series of forms to ease planning of adventures. You just list the key scenes that you want people to hit, with the high points of the scene.
Be prepared to run with wild directional changes in the event of catastrophic failures. You can still direct things to your key scenes, but you'll take a different route. And that's a wild and crazy amount of fun. Check out my last game session for a great example:
http://www.rpg-campaign.com/planner/streets-auburn