Lise's 5 C's
If important concepts are missing from the index, users assume they're not in the book.
Consistent depth of coverage and phrasing.
Logical structure throughout.
Parallel structure for parallel entries.
As long as formatting and punctuation are consistent and clear, the specific style isn't important; it's often set by the publisher anyway.
Each entry's purpose and scope is obvious and clearly unique from its neighbours'.
Phrase entry to include necessary links between and roles of terms.
Entries should be easy to scan visually.
Avoid too much ink on the page.
Just as in poetry, Less Is More.
"See"references lead users from synonyms to the terminology used in the book, and/or to a place in the index where the information is concentrated (to save space or for consistency).
"See also" references connect related concepts and information, especially broader and narrower terms.
Think like a thesaurus: you're building an index language.
Useful phrases real users are likely to look up
For every entry, think: would someone look this up? why? how else would they probably look it up?
Indexing is like Jeopardy: what questions does this page answer?
Avoid terms that are too broad, or non-descriptive out of context, especially as main entries.
Never use these as main entries: About, How, Introduction, Overview, Using, What, Why (etc.)
Watch out for vague verbs or "orphaned" single adjectives as main entries.
Multiple access to each concept
Rotate all useful words, so that each appears as a main entry, and each has all locators.
For every concept, think: what is it? what is its name? what does it do?
Mirrors the author's terminology on the page referenced
Users are visually scanning the referenced page for something resembling the index entry.
They are not necessarily reading for comprehension, so don't paraphrase the author much.
Provide "See" cross-references from "real world" synonyms to author's terminology.
Break down main entries of more than 5 - 7 locators into subentries, to distinguish contexts. (People can't remember more than 7 things, and we have only 10 fingers to stick in a book.)
Separate very disparate contexts with subentries, even if there are only 2 locators.
Consolidate multiple subentries into one main entry if they all lead to the same few locators and the concepts are not logically distinct (i.e., don't spit hairs.)
No "passing references"
A concept must be presented with a sufficient amount of useful information to make it worth the bother of following an index entry to that location.
Locators must be accurate.
Print page references should include page ranges, generally of no more than 10 pages. (Ranges tell users the size of the information at each location.)