DRC raised an interesting point about agents (and as my computer doesn't allow me to reply to comments ...) so I thought I'd touch on the subject here.
In this world of self-publishing it's probably tempting to think, fuck it, I'll cut out the middleman (middle-men if you include publishers in that definition), stick my stuff up on Amazon and, eureka, I'll be an over-night sensation. But whilst I'm sure for some people that's a strategy that works, for the vast majority it doesn't and they languish un-read, un-discovered and un-paid in the very crowded e-backwater that is 'self-publishing'.
My own belief is that once you have a book you think worthy of being published then it's worth at least attempting to get yourself an agent. A good agent will have three invaluable abilities: he or she will have an instinct for what will sell; will be able to advise what should be done to your novel to make it sell better; and will know the people who might be persuaded to buy the bloody thing. And to this list should be added that they will be a font of good advice (and believe me you'll need it) and a receptacle of oil to be poured on troubled waters when you get really ticked off with your publisher.
I came to John Jarrold by simply Googling 'Literary Agents + Sci-fi' and then checking out what came up. In the end there were three agents I thought seemed promising, checked out their submission guidelines (please, please do this before you submit and follow them, otherwise it's manuscript interfacing with bin time) and sent my stuff off. John came back and the rest as they say is history. And that history is that whilst I've been a client of John's (three years now I think) I've become a better writer. The whole agent/publisher complex is designed in part to provide friendly criticism for a writer and such criticism is vital if you're to improve (of course, being given money to write also helps!).
Criticism is part and parcel of writing but whereas some of the critics out there have got issues and agendas, your agent will be constructive and honest.
So ... my advice: before abandoning the well-trodden path of getting yourself an agent at least give it a shot.