Doing BDD/TDD along with a good text editor/IDE and learning the keyboard shortcuts will help. E.g. in vim with the Rails.vim plugin you can switch between controllers models and views very easily or go to the file inferred by what is under the cursor. You can also navigate back through your history too which can help.
I may typically go from a cucumber feature, to step definitions, to the config/routes.rb file to a controller spec, then a controller, then a model spec, then a model, then a view, and cycle through that a few times. The cucumber features help to maintain the mental model of the highest level of what I'm doing then the lowest level might be the implementation of a method in the model.
Each step you're going from the business requirements end right through the full stack of the application until you're at the lowest level of logic. And what you should be focusing on is guided by failing tests and making them pass. E.g. to pass the feature you need a certain action for a controller which then needs a certain method on a model.