Like any unwanted habit, all you really can do is to re-train yourself. Read slow on purpose, if you catch yourself skimming, start over. Maybe it's possible to condition yourself (rewards for not skimming etc.), but I don't know.

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I start to do it, when I'm either bored or the exact opposite, hyped for what's coming. I don't mind the bored skimming. Because of the other case I really started to like audiobooks, they force me to a certain tempo that's far slower than my normal reading speed and it's impossible to skim ahead.