I've always written novels, but there was a point where my whole technique fell apart on me, and I essentially had to relearn how to write a novel. This is the core difference for me: In a short story, you write about a single event. An incident. One concept. In a novel, you explore a premise.
A premise is a concept that could go in all kinds of directions and turn into a million different stories - what if there were vampires, and you fell in love with one of them? That's a premise that could apply to half the vampire books out there. The kind of book you get results in how you personally explore it.
The other important thing about a good premise is that it gives you some obvious steps to work with. Take the vampire premise. You already have at least two main characters (vampire, MC) and some basic plot points: MC has to meet vampire, fall in love, discover person is a vampire. Then it has to get complicated somehow. What about rival vampires? What if the vampire is good, but others discover it, and begin a witch hunt? More plot points ensue: MC tries to hide vampires identity, vampire gets discovered, runs, perhaps gets captured, MC must find a way to help it escape.......
A good premise will make a whole novel unfold for you as long as you keep asking "what if" questions and exploring the idea. You can replace a premise with an idea or theme - what if humans all became barren - or - racism. But if you're struggling with writing a novel, I wouldn't suggest using an idea or theme as the basis of your story. It presents less starting points, which makes it harder to start creative snowballing.
I know I'm going on and on here, but let me add one final thing that should help in writing your novel. When you do come up with a premise, if you find yourself getting stuck, just start asking yourself things like: so who would the antagonist be? How could I complicate this? What's the most horrible thing that could happen at this point?
Try to find yourself a beginning (inciting incident followed by suspicions, discovery, then understanding of idea) a middle (complications, antagonist steps up, things start falling apart/going wrong, worst thing of all happens) and an end (final desperate effort, bitter struggle, succeed, aftereffects.)
Okay, I lied. One MORE thing. If you ever find yourself getting stuck or running out of ideas, find someone to talk to about your story, or even post a thread about it here on AW. Talking about it verbally will stimulate your creative juices and if you're anything like me, will help you start brainstorming all over again.
Phew. Hope some of that helps.
-Feathers