There's another option besides mixins and base classes, composition. You can break your fat model into lots of smaller classes that you aggregate into your original model. The big advantage over using mixins is that the scope is kept much smaller. Keeping scopes small in general is always a good way to scale up to a large codebase (functional languages get this pretty much for free).
Yes, I generally stick to composition. "High level" classes end up using a bunch helper classes to perform specialized tasks. I find that I do not often find the need for an abstract base class or mixins, but I will use them if it makes sense.
