Expanding on my comment, some word types that are likely to be written in kana which haven't been covered so far:
Cases where one or more kanji in the compound are considered rare/difficult (for the level of the text).
Examples: 石鹸【せっけん】, where 鹸 is the sticking point. This is commonly written せっけん or 石けん, or if the kanji are used furigana may be provided.
Cases where the kanji are common but being used for sound rather than meaning.
Examples: 沢山【たくさん】, although it is seen both ways, 薬缶【やかん】, where the kanji 薬 is in there for historical reasons but doesn't much related to the modern meaning, and 駄目【だめ】, which is also often written in katakana.
Cases where there are multiple kanji options, particularly common for verbs.
Example: わかる which can be written 分かる、判る、解る。 The different kanji have slightly different nuances, but if you are unsure which is best you can get around it by using kana.
Cases where katakana are used by convention, such as in scientific contexts.
Example, ゾウ科【か】 for the family Elephantidae, instead of 象科。
Possible combination of 1. and 2. above: 綺麗【きれい】 uses the non-jouyou 綺 so an alternative is to write it with another, similar kanji with the same reading - 奇麗. However, 奇 doesn't fit well with the meaning of the word, and it's not uncommon to see kana used.
In some cases the choices are down to personal preference, but also audience (if you are writing for children or adults, for the layperson or the specialist), and how you are writing (people tend to use more kanji when typing than when handwriting).
Example: 歳・才【さい】, for counting ages. 才 is a grade 2 kanji, so only books for very young children would use kana. 歳 isn't taught until much later in school, though, so 才 is quite common, even though official documents will use 歳。 才 is also more commonly used when handwriting - not necessarily because people can't remember how to write 歳, it's just quicker. (see also this question )
