I try reading as many books as i can. Movies ,the boards and almost everything, can give you great ideas.
So, i incorporate these things ,as much as i can, in a general plot and do everything else on the fly.
This has many advantages (as mentioned ,freedom for pc's and non-linearity) but this style greatly depends on the dm's mood and his eagerness to dm.
It is so much easier to dm a group of players when the biggest part of the story is already designed, even if you feal out of place and not in the mood to do it.It is difficult to come up with interesting ideas sometimes and this can easily spoil the session.
I ve seen that:
a)This style of dming can be difficult for someone new in the game, with little experience
b)It requires lot of energy and dedication from the part of the dm
c)If dedication and experience exists the results are almost always rewarding
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The wizard