A lot of the above suggestions come down to personal preference, and what works within your particular group. However, I'm going to give some fairly uncontroversial advice that every DM should take to heart.
Take the time to know everything you can about the PCs at your table.
|And I do mean "everything".|
Minimize The Monkey Wrenches You Have To Deal With
It might sound like a bunch of busy work, but trust me on this, it's not. Before you begin your campaign, review every character in the party. Look at their attributes, check their skills, and review their feats and class abilities. Look at their spells, and go through their backpacks. You don't need to know them like the back of your hand, but the more you know, the fewer the problems you'll have later. You should also review the characters every time they get new rewards, buy upgraded equipment, and every time they take a level. Sit down with your players, and ask them to explain their characters, and attributes, to you to ensure you're on the same page.
Because you have to know what you're dealing with.
For example, say your mid-level party doesn't have anyone with trapfinding, and no one has invested a lot of points into Disable Device. That's something you should know before you make them crawl through a dungeon where, if they aren't disabling the traps they find, they're going to spend 3/4 of their resources healing from the damage said traps are doing to them. Or, say you wanted to throw a challenge at your spellcasters, so you give them some monsters with spell resistance. That's a good thought, but if the casters have to roll a 19 or 20 on the die to beat the SR, then the "challenge" feels more like a cudgel the wizard and sorcerer are getting hit with.
|And that's before they even pick up a die.|
Then there's the opposite problem. Rather than making a challenge that's too difficult, you make one that's too simple. The mystery of who killed a room full of victims is a lot easier to solve when you can talk to the dead, or simply ask questions of the divine to confirm your theories. A group of swarms would be a problem for most parties, but you have an alchemist, and an evoker in the midst of a turgid love affair with area of effect spells, so they're unlikely to last more than a round. Perhaps you'd planned on making the climb up a decrepit clock tower a central challenge of the next session, forcing the party to make skill checks while dodging falling bells. Of course, if the whole party can fly, it sort of renders the whole thing moot.
It's a simple rule, but definitely worth remembering. Because you don't want to be halfway through what should feel like an epic session, only to have your story completely undercut by the fact that everyone in the party is immune to the poison your big bad relies on, or to have the whole party die in what was supposed to be a warm-up because you misjudged what the "average" hit points among them was by about 45.
That's all for this week's installment of Moon Pope Monday! Hopefully it helps the DMs out there, and makes your games easier to run. If you want to keep up on all my latest releases, then follow me on Facebook, Tumblr, and Twitter. Lastly, if you'd like to support Improved Initiative, all you have to do is go to The Literary Mercenary's Patreon page, and become a patron. $1 a month is a surprisingly big help, and there's some sweet gaming swag waiting for all my new patrons once you've signed up!