One technique I find particularly helpful is keeping a text document of changes I plan to make. Then whenever I have some spare time, I open the file and read the list of things I need to add to the game. Little by little I implement each change, but the moment I remove it from the to-do list, two more things get added on! Sometimes it feels like I'm making NEGATIVE progress...!
It actually reminds me of what happens when debugging my programming homework. When you fix one error and get a whole list of new ones -- that's when you know you've made progress! All that remains is to survive each wave of errors until the final product works as expected.
And that's what I plan to do here.