Does anybody have a good technique for coming up with original material?
i don't really know if there's a substitute for imagination. just think about what you would find fun in a game. what kind of stuff would
you like to see/do in a game.
maybe you could play other peoples maps and figure out things that would make them better/more fun, then work on incorperating those ideas into a map of your own. getting to know the half-life entities also helps in giving you ideas for implementing them in novel ways.
sometimes i find that when i lay in bed and am drifting off to sleep, if i think about my current mapping project, i can come up with new ideas...
good luck