Section 1: Players, and how to deal with them.
1: Always know your players!
Sadly, this technically counts as "planning", but you really need to learn the personalities of your players. More specifically, learn about the things they like to do with their characters. Do they cast spells from a distance? Do they move to the best location to grab all the Attacks of Opportunity they can?
After you learn to observe behaviors, try to get quicker at it. If you can tell how a player will do things from the moment they first say "Hello", you're doing it right.
You may ask yourself how it's possible to catch on to a person's characters that fast. You don't want to stereotype, but consider these traits:
The player speaks quietly. (Their character is likely a spellcaster or rogue. Something that lets them stay in the background without drawing much attention other than praise. They may just be trying to be cool, too. If so, their character is probably a loner, and hangs off to the side until the party realizes they need him.)
The player likes to be loud. (They want to be seen, a lot. They likely have some type of tank-ish character, who's right in the fray. Maybe they play a spellcaster, probably an evoker, who blasts things for insane damage.)
The player waves. (Maybe they don't like the idea of getting close to people. Will they fight from a range?)
The player shakes your hand. (They're either really excited, or just polite. If they're also loud and their handshake is particularly firm, they're almost certainly a melee type. If they shake hands for the sake of being polite, they're probably going to be very knowledgeable problem-solvers and will take their time planning their actions. They're likely methodical or crafty.)
The player is all dressed up as their character. (First of all, you know what they're going to play the moment you see them, so that's a big help. But it's also a hint that they know exactly how they're going to play. They won't need much help on the rules, but it's guaranteed that there will be a problem if they see you doing something they don't like. Without a doubt, they're going to RP, and they'll expect everyone else at the table to do the same.)
The player is casual. (They may still know what they're doing, but they're more likely to need a little help with the rules from time to time. Unlike those who are dressed up, they're probably not so worried about RPing, and are less likely to care if things don't go their way.)
Obviously, there are exceptions to these, but if you practice, you should be able to be right at least 50% of the time.
2: Deal with crap!
Unfortunately, not all players will see eye to eye with you. You can have all the angst towards them and their stupid Drow ranger as you want, but don't let it show.
When you're the DM, you're expected to be the mediator and the storyteller. You don't need to make everything make sense, but you do need to tell the freakin' story. You may feel that some players get in the way of this, but they shouldn't.
You're the DM, and you're all-powerful. You could blast the player all to Hell, but instead, why not use your DM power to take care of the problem the less painful way? This differs from situation to situation, so it will take some practice to learn to do it, but it's certainly worth the effort.
((More to come.))