Knowing When to Refactor

You're in the middle of implementing some new functionality when you stumble across some poorly written code. Fixing it properly will take some time and, for the stuff you're working on, you can get away with making a couple of tweaks. What should you do? Refactor it now? Or leave it for another time? If so, when?

Douglas Squirrel told me about a neat solution that I hadn't heard of before. The idea is that the first time you come across some smelly code you write a comment that says FIXME. Each time you encounter the code again, or someone else does, an extra X is added so it becomes FIXXME, FIXXXME etc. By the time the comment screams FIXXXXXXXME, you know what you have to do!