Doing it in your head is hard, but if you a great memory or can write things down on paper, then you could:

1. Generate a bunch more "random" coin flips than you need and write them down. Do your best to be unbiased, but don't sweat it.

2. Start a new separate list of results.

3. Go through the original results a pair at a time. If the pair is "head, tail", write "head" in the new list. If it's "tail, head", write "tail". If it's "tail, tail" or "head, head", discard it.

This is basically Neumann's "fair coin from a biased coin" trick:

https://en.wikipedia.org/wiki/Fair_coin#Fair_results_from_a_...

In theory, since you know the trick, it might influence the way you generate your initial random sequence. In practice, most of us probably aren't that smart. If you are, repeating the whole process using the results of the previous run a few times should guarantee enough complex mixing that your brain isn't clever enough to back-propagate the trick into how you generate the initial random coin flips.