Paul McAuley runs you through the basics:

Traditionally, SF heroes solved problems by application of intelligence and scientific knowledge. These days, you can substitute lasers or AK-47s for scientific knowledge. Or swords. The equivalent of the internet or mobile phones are used only when the hero needs to find something out. Usually someone else does the actual typing. Don’t include any science that might frighten the readers.  Anything found in SF written before the 1980s is usually okay. Nanotechnology is basically magic. So is genetic engineering. Also quantum mechanics. Virtual reality is more or less the same as a video game. Planets can be treated as a single country, with uniform climate and culture, and no more than three unique features that distinguish them from Earth.

Very funny, with a slightly bitter aftertaste.