"Programmer's block", like "writer's block" is an absence of creativity -- you can't figure out what to write next, not because you don't know how to implement X, but because you can't come up with any ideas for what X should be.

There are two solutions to this. First, cultivate a good source of ideas. Beta testers are wonderful for this purpose. Second, don't start coding as soon as you have an idea -- write them down, and when you don't know what to do next, go back to your list of ideas. I aim to have 1-2 months of ideas for tarsnap planned out at any given point, so that if I'm feeling uncreative I can just pull up my coding to-do list and start implementing things.