One of the most useful items in my bag of tricks is learning how to make things modular. If I put together a dungeon, and the PCs end up going on a completely different tack (as PCs so often do), instead of either scrapping the dungeon entire, or trying to find a place to fit it in later on (although I do the latter on occasion as well), If I make it fairly modular I can take various bits of it and insert them into whatever adventure bit the PCs do happen to go for. Take a maze out of a dungeon, swap out some of the monsters, and it can become the overgrown garden/hedge maze of the abandoned noble's estate. A thriving temple of the local deity that the PCs never explored beyond asking the nearest cleric for information could become the ruined temple that the PCs later explore. For that matter, most temples or noble's mansions will have the same sorts of rooms in them. Change the shape and the layout and you've got a nearly instant building you can drop into an adventure. A mage tower with different furnishings could become one of the towers in the evil king's castle. I also found that learning the tactics of a few different types and CRs of monsters is helpful. If I need an encounter that I didn't have planned out in advanced, I can throw in something I'm familiar with and merely have to roll up hit points for them. Thinking on the fly, and staying one step ahead of the PCs is rather important to me in how I run a game, and these sorts of tricks help with that.
Another one that is handy for me is to keep track of the encounters in general lumps. The DMG says that 14 encounters of an equal challenge rating to the PCs' level gives sufficent experience to level. Learning how to guage how different level CR encounters fall into that helps as well-- so the 4 level 1 encounters would be about the same as one level 3 encounter for the level 3 PCs, for instance. I keep tabs on what the wealth-by-level chart says the players should be at, and if I have a series of unplanned encounters cause the PCs went a different direction than I planned, it's easy to give out treasure based on the number of encounters. So for every 3 or 4 encounters, they would have roughly 1/4 of the treasure they'd have by the time they reach the next level. Saves me having to spend a lot of time rolling up the treasure values for random or unplanned encounters. If I know the PCs should have a total of 10,000 GP worth of gear after a set of encounters, they may find a few gold pieces or masterwork items on most of the critters, and then perhaps a chest that has a valuable item or two like a ring of protection +1, or +1 crossbow; perhaps goggles of minute seeing, or whatever. And then gems and coin to bring the total to where it should be.
Anyone else have handy little tricks they keep to hand to make their DMing better for the PCs and easier to manage from the DM side?