I've only been writing book-length work for about 7-8 years so I haven't experienced being blocked (yet). I was probably blocked before I began to allot regular time for writing. At that point the notion of a long work seemed so daunting! I find that regular practice helps keep me thinking and plotting and churning out pages. I also use the tactic of diagramming a story's beginning, middle, end, and some turns in between. Though they might become moving targets, at least they are targets to write toward.