Make a to-do list of very, very specific small things.
If something on the list can be broken down into smaller steps, it should be.
Take it down to the point where gathering each thing necessary to complete one task is a separate step.
Sometimes that helps.
My biggest trap is when a decision needs to be made, there are just too many possibilities, and I want to make the optimum one.
If I put it off, the answer eventually just comes to me. But I think I've grown to do this too often and as a result end up waiting for the answer to magically appear instead of working on a solution. Sometimes you just have to pick one option and run with it.
Right now, I'm not making any progress on my driving cabs because I have the parts to do the controls up to four different ways and can't commit to any of them because I'm afraid there might be a better way out there that I haven't thought up yet.

I also have this thing where once I know something works, I lose interest. I like the challenge of figuring it out and making it work, not the menial task of finishing it.

What has helped me recently is dropping the number of projects. I've sold/thrown away a lot of stuff that was squirrelled away for "someday" projects. Without all those in my head overwhelming me with things to think about, I've been making more progress on my main two projects (linked in my sig).
I'm shooting for having the stand up cab done by the end of the year, which is a year later than I'd planned.
I'd like to have the driving cabs done by spring. We'll see.