I haven't tried it yet but it looks good. Take a look over at Justin Achilli's take on the Kanban Method for GMing in this article and this one. Basically it involves keeping track of whether your encounters / adventures are still in the Planning, Foreshadowing, Ready to Use, or Re-Visit stage. Perfect for those more sandboxy adventures!
Me, I normally just sit down and list out the grimy Left Overs from the campaign to date (always best to clean them off the plate early before I get distracted by new stuff) and the too cool Hooks that have already been foreshadowed (some of which haven't been fleshed out but seemed a good idea at the time). In most sessions of an open-ended sandboxy campaign, I write them down to ensure that there's some closure from the Left Overs and some more juicy progress on the old Hooks. In my more episodic Demon campaign, I do it ever so often when it's getting backed up or when the players have done something that's too full of delicious repercussions.