Fixate on a number fairly late in the sequence (the millionths place seems to work well). If that number is 5-9, push 'f'. Otherwise, push 'd'. Keeps it pretty consistently around 0.5.
> In a class I taught at Berkeley, I did an experiment where I wrote a simple little program that would let people type either “f” or “d” and would predict which key they were going to push next. It’s actually very easy to write a program that will make the right prediction about 70% of the time. Most people don’t really know how to type randomly. They’ll have too many alternations and so on. There will be all sorts of patterns, so you just have to build some sort of probabilistic model. Even a very crude one will do well. I couldn’t even beat my own program, knowing exactly how it worked. I challenged people to try this and the program was getting between 70% and 80% prediction rates. Then, we found one student that the program predicted exactly 50% of the time. We asked him what his secret was and he responded that he “just used his free will.”
But seriously, the intuitive concept of free will, something like "unconstrained choice" leads to the idea that free will means simulating randomness - but once you're there, it looks more or less like nonsense - "acting randomly"
I was doing better by just trying to continually break whatever pattern I had fallen into, and occasionally throwing in some long predictable patterns (i.e. all the same key) to simulate the low chance of a long predictable string occurring. That had me down in the ~.4 range for a while. (Edit: Down to 0.23529 this try)