Joel points out that the hardest thing for him is the actual 'getting started', and comments on the constant moving forward of 'fire and motion' - I think there are two separate problems here, which are faces of the same coin.
Productivity is all about motivation. If you're not motivated, you can't be productive. Motivation is what makes you open the editor. Working on new code is cool, it's motivating. You can code for hours on new code. Perpetual re-invention and re-factoring, to make old code new again, gives you that motivation to continue working on it.
However, it's not solving the problem of motivation per se, since there are a number of ways to become 'motivated'. Eventually, code might become stale, and if the only way you can be motivated is to work on new code, I don't see what is going to motivate you to open the editor to refactor the code to make it new again. While moving forward is a good tool to motivate yourself, you need other tools in your box too....
The way I see it, you sometimes need to force yourself to open that editor. Even though you don't want to, you have to open it and attempt to concentrate on the task at hand. This will be entirely unproductive for the first hour, maybe the first few. However, it gets you back in the groove. It becomes easier again - but you just need to get over the initial hurdle. Teaching yourself to do that is hard at first, but possible and definitely helpful.