It depends on whether I'm starting a new game, or if I'm running a continuing session.
New game:
1. Sit down with the players and create characters, histories, and kickers. Kickers are little hooks that the player gives me that starts the session. I.e. Bob finds his neighbor hung up in a shower, skinned. or Sara just learned that the bartender knows a guy with a big tatoo of an otter on his face. The same tattoo'd man that killed her mother.
2. Spend the next week creating a tree that connects various kickers to plot ideas I have. Create 3-5 encounters of varying difficulty.
3. Erase everything I had previously come up with, and start again.
4. The first night of the game I set everything up that I'll need, and run through a game-opener.
Continuing sessions:
1. After each game, we have a few questions that we go over. Stuff like, did you like this? What was funny, interesting, unique? I take all of that, and use it to fine tweak the next session. For example, one session dealt with a couple hazards like swimming, smoke dangers, and climbing. A player said that didn't feel very enjoyable to spend 20 minutes of rolling and failing checks repeatedly to cross a 20ft river. I now try to limit the amount of "natural" rolls made in a session.
2. The next few days I usually look at things my players missed in the last session, and what, if anything, can be tied into an over all plot. More often than not, it's my player's mistakes that create the larger story lines.
3. I spend no more than a few hours looking over encounter creatures. I do this for two reasons. One, because I know most of the statistics of any monster in the MM, and two, I really only need the CR, and general info about a creature to setup the beginnings of an encounter.
4. The day before the game, is when most work gets done. I have this day off, so I often gather books needed, print out monster stat sheets (I have a really nifty one I got from somewhere), and draw any necessary visuals out on graph paper, to be quickly transferred to the battle mat later.
5. We game.
Ktulu