I keep a master list in one of those 4x6 vinyl 'brag book' photo albums from the dollar store. Each photo pocket holds a list of measurements for each room (i.e. windows, rugs, lamp shade sizes, etc including details such as left or right hand swing for doors) The last two photo pockets hold lists of wants & needs. The album lives in my backpack/purse which I never leave home without.
It's saved me numerous times. Plus, if I stumble across an item on my want list which is deeply discounted, the measurements are at my fingertips and I don't miss out on a great deal. It's annoying to get midway through a project and discover you forgot a necessary item. It's doubly annoying to be forced to postpone completing the project because picking it up requires a 50 mile round trip because the local hardware store only stocks the bare necessities. No more. I am prepared.