Seems to me there's two ways of doing this: 1. Totally clear your mind of the previous letter and make each letter a separate decision. That way you only have

Seems to me there's two ways of doing this: 1. Totally clear your mind of the previous letter and make each letter a separate decision. That way you only have to avoid being biased toward using one letter or the other too much. This is probably pretty hard. 2. Allow yourself to be aware of your previous answers, but thwart your own biases by including patterns that cancel them out. I went with the second option and consistently got between .45 and .55. I know our main bias around randomness is not realizing how many runs and patterns exist in truly random data. So I occasionally included runs like "fff" and "dddd" and, rarely, patterns like "dfdf" and "fdfd."