To avoid "memory overload", I have taken to literally ignoring parts of the system which I currently don't need. I.e., I don't even look at other classes' code before I don't need them. Else, I would have to spend 3 days of understanding it all. That's for very big programs, and of course I do look at the overall structure and what patterns have been used to couple classes together. But if a method promises something with a contract, I won't read through it, but treat it as a black box. My theory is that cognitive power is like money spent during the day. I can recharge it after a few hours by taking a nap of 20 minutes, but I better watch what I expend my cognitive credits for. And spending it all on reading other people's code (which has probably already been revised 10 times) is not worth it.
...this is what we all try to do I guess. You said "I don't even look at other classes' code" but I always find it much easier to not look at a function's code or at the code of a method of an immutable object or a "predicatively mutable" objects than for classes of highly mutable objects.
...that's why I'm currently investigating functional programming as way to make it easier to hold larger parts of programs in your head.
