I'm sure this sounds disappointing, but basically there's nothing you can do other than start. There are several ways to trick yourself into starting, though. Some random ones:

* Break big tasks into small pieces, make a list.

* Visualize yourself taking (and completing) the first (physical) step.

* Take a time based goal instead of task based one, e.g., just spent 5-10-15 minutes on the task.

* Start with big rewards (for small things) and try to make them smaller as you go.

* Set deadlines, plan backwards (but don't abuse it to procrastinate ;)

I think of discipline as a muscle that you have to train. Don't expect to be able to lift a complete weekend of productive work if you usually 'slack off'. I definitely recognize your behavior.