Teach students how to choose keywords. Mind maps are based on brevity, just write down what the brain really needs to support memory / understanding, nothing more. The ability to choose good keywords is therefore absolutely central. So what is a good keyword? It depends in part on context and individual, but two principles always apply:
(1) are a bit unusual (therefore usually nouns or verbs)
(2) makes you remember a piece of text
A fun way to practice the first principle is to let students take out "bad keywords" in a text, ie ordinary boring words like and, one, but, I… (the reverse method always works). Have students rehearse why the words are bad ("because they are common boring words").
The second principle you practice through comparisons between possible keywords (there is rarely a given word). Start by modeling how you think when you select keywords from a text on the board. Then have them extract keywords in a short joint text individually which they then discuss in pairs. Different opinions are good, it leads to conversations about word choice. End with a whole class discussion about different alternatives.