I really only have one good tip for this. Get off your butt and write. Sometimes you write something really amazing (or at least you're tempted to think so at the time) and it makes you feel all better. Or at least marginally better. And then you feel all shiny and happy and prolific, or maybe you just wrote three pages but it fit two really great pieces together that didnt quite turn smoothly--kind of like cartilage. You need those pieces of cartilage for your story to flow. And then, even if you only wrote that 3page piece of cartilage, at least you know your two really great scenes now flow smoothly from one to the other.
When I get depressed/hopeless/think what's the point, I'll never even find and agent, let alone a publisher, the last thing I want to do is sit down and write my story (after all, it's probably my story's fault that agents tend to ignore my queries, right?) But if I suck it up and force myself to write, I generally feel a little better about my story at least, if not about my prospects.
Speaking of, I should probably go find some more agents to annoy...