What would you do if you could only do ONE thing?
Write the living daylights out of your book.
It's the first, it's the strongest, it's the ONLY rule. Write what takes YOU by the throat and shakes you. Everything else... is ephemera.
And, on a somewhat different though related topic, there's been a lot of stuff written about the essay by Norman Spinrad about, oh dear, "third world" SF (here's the original, and some other places to read up on reactions to it are here, and this one)
I would also, however, like to offer this piece
Feel free to discuss. Play nice.