A question for you elder DMs out there: How do you keep youselves organized when DMing a game?

DM screen is a good tool, 3 way folded card with useful tables on the inside. Buy one (~$10) or make your own.
If you know the party pretty well and know your monsters you can sketch out a quick "what A, B, C.... needs to hit monsters ....X,Y,Z matrix.

3x5 index cards are a must! Have one set with your monsters and treasures on them and another with your room notes on them. Stack and spread them out so you can quickly switch as you have to and insist that you really do need that much table.

oh and if you're like me and can't afford a screen, use two folders and stand them so they overlap slightly

A screen is a good idea, don't let them see what you are up to. I'm a big fan of the index cards. Some with monster stats and some with treasure on them. I print out my maps, one player copy and one DM copy. I then use the cards to keep track of all my notes on the maps.

In addition a "what if" tree help keep the plot going. You are never going to think of everything your players will do but try to cover the major alternatives.

Also have the "info notepad" read to write down name and places the PC see or interact with. I then transfer this info over to my master list after the adventure. This way you can build on things and not forget details for the past.