How do you write large programs in dynamically typed languages?

I've written a few medium sizes (500-1000) line programs in languages like bash, python, Ruby, and JavaScript. I use these languages because they lends themselves towards scripting (bash, python) or because I'm forced to (Ruby, JavaScript).

At first things are alright. In small programs I can keep track of types and what should be passed where. When programs grow and I try to refactor (50% of my time programming in Java is spent refactoring to keep my programs clean), or when I try to add a feature after taking a several month break, I find myself completely lost and unable to easily make changes.

So, how do you write maintainable programs in scripting languages?