Q: What should be the format of an adventure?
A: Each adventure should have the Begin-Middle-End cycle. Sometimes you may have several of these going on at once. It is best if only the beginning and the end overlap, to keep things clear and fluid. Follow the format of books:
Q: At what point should I end the game session?
A: It is best to end it at a point where the characters are in a "static" position (if you are in the middle of a combat, then a lot of information has to be carried over to the next session), for example, when the party is camped. Sometimes cliffhangers work, but these should be done at a time when no extra paperwork needs to be carried between sessions (i.e. before the big fight, not during), at a point where characters will have time to "get in character", and make sure all the players can turn up on time.
If you have to end a specific time, then make sure that the party will be in a "static" position before that time, and end it there. This will avoid "Oh, just a couple more minutes.." situations.
Q: What subject matters should I avoid?
A: Fundamentally, everyone should enjoy the game. If there is a subject matter that some players find uncomfortable, then avoid it. If it does not further the plot or improve the quality of the story then there is no real point in using it. I personally feel that games should be about the same morality level as a UK cinema certificate 15 (I don't know what that is in the US).
Q: What can I do to improve the atmosphere of a game session?
A: There are many "devices" used to do this:
Q: The characters are going on a long trip, what can I do to make it interesting?
A: You must remember that you do not have to role-play everything, just as books don't mention every detail ("as Conan flushed the toilet.."). This means that if the journey is not an important part of the plot or will simply "get in the way" it is sometimes just best to describe the trip with a paragraph or two, e.g:
"The trip lasts fourteen long days, the weather maintaining a steady and dreary downpour, making the whole journey miserable. The travelers who crossed you path where similarly depressed and declined to talk. Finally, with great pleasure you can see the walls of Kear-Thaleth in the distance."
Usually however you may wish to role-play some of the events of the trip but again, all of the "minor" events need not happen. Some of the events may be part of the plot, some may be irrelevent but planned, and some may be completely random (the DMG and the MCs provide charts for random encounters). Make sure you know what the parties marching order is (this can be important for ambushes), as well as who keeps watch, and when at night (for any night time activity).
If you roll up the random encounters before the session it gives you a chance to give you a little background on the situation, for example when they bump into the orc party what are they actually doing and why (are they cooking, walking, surprised, do they see the party before they see them). A sample of events will often go something like this:
"For two days the journey has been fairly routine, the warm weather has
been constant and the travellers you have encountered have been friendly
but intent on completing their trip. On the third day, as you set off
after lunch, you
[points at a player] notice some blood off to the side
of the trail."
Occasionally you might like to mention non-confrontational events that occur during play such as a flock of geese flying over-head in formation. Weather should also be mentioned, particularly if it is non-seasonal.
Sometimes you might like to get the party to come up with their own things to pass the time, for example, they might want to go hunting deer.
Q: Are there any other methods to DMing, other than 1 DM and 1 player per PC?
A: Yes, there are several, these include: