I've blogged before (here for example) on the writer's habit of prevarication. Almost everyone I've ever spoken to who writes admits to it. They'll put off starting writing as long as possible. So how to get round this?
If I've got a particularly bad case of the avoid-writing-itis I tend to trick myself by starting with the easy stuff. Something that's still part of the work in progress, but is easier to do than the bit I had been intending to start on. Then, once I'm into the writing process I can switch back to what I should be doing. Once I'm writing, it's easy to keep going.
The book I'm working on at the moment makes this particularly easy because it has 100 self-contained sections. Some are harder to write than others - so if the next one on the list really strains the brain, I'll pick off an easy one and then come back to the tough one. But even writing something more continuous, it's usually possible to skip around a barrier that's encouraging prevarication, get yourself up to speed, then come back and smash that obstacle.
It won't always work - maybe won't work at all for some - but I find it useful.