You can ignore much of my advice if you're looking at being a hacker-slasher GM. I'm a creative person who likes creative things. The only bits that really apply to hacker-slashers are the last three points.
1st. Broaden your knowledge base. Read fiction (of all genres) like mad, watch movies, watch TV. Read philosophy, read up on the sciences, read politics (especially for Shadowrun). I have everything from Nietzche to Stoker, Lao Tzu to Thomas Hobbes on the shelf above my Shadowrun books. There's also the Divine Comedy, the complete works of Edgar Allen Poe. I've studied biology, chemistry, physics and engineering. I've acted in plays and taken acting classes, studied art history and musical theory, played the oboe and marched with the sousaphone. I'm not trying to trumpet how awesome I am, I'm not really that interesting, I just study what I'm interested in. What I'm trying to say is, the more you broaden your knowledge base and expand your horizons, the more you bring to the table as a GM.
2nd. Get creative. Start writing, short stories, thought exercises, world-building, plot outlines for campaigns. Anything to get your mind more flexible and encourage on-the-fly solutions.
3rd. Learn to read people. If you can't tell when people understand you, when they're bored, when they're happy. What frustrates them, what motivates them, then you can't be their GM.
4th. Learn the rules. Read the books, cover to cover, get engaged in the rules discussions you see on this forum and others (dumpshock is a good one).
5th. Be confident in yourself. It sounds stupid, but if you aren't confident, then you will fold to any pressure offered, your villains will come off half-hearted and rules-lawyers will walk all over you.
6th. Try to find a simple, straight forward module, and run it. See what happens.