What do you need to be a good indexer? You need time and patience. You need to switch off the media, ignore e-mail, and concentrate for long periods, because you are, in a sense, reading and re-reading the book in question more closely even than the authors. You need a sense of proportion, such that a single passing reference to this or that does not cut the mustard as regards the special form of acknowledgment that is a discrete entry in the index. You need an excellent library of reference books (above), trustworthy ones—none of this wiki nonsense. You need a good chair, a strong bottom, broad shoulders, and a wide desk. You need to care enough about consistency to be able to render at times vital, last-minute assistance to overworked copyeditors and typesetters, but also to be flexible enough to allow for subtly revealing forms of creative inconsistency within the parameters of the index itself. You need to be a good note-taker, and to have an eye for detail, as microscopic as possible, whilst keeping hold of the larger vista. These are not insignificant skills, and it is a tragedy for this little profession, this calling, that publishers are no longer in a position to pay fair market rates for the invaluable service an indexer provides.
In the end, a further factor may strengthen the usefulness of an index: luck. After the job is done, and the index itself is read for the usual editorial double-checking, one suddenly becomes aware of arbitrary but stimulating adjacencies, a simple consequence of alphabetical order. Thus:
Morgan, John Pierpont (1837–1913)Morgan, William de (1839–1917)Morland, George (1763–1804)Morrell, Lady Ottoline (1873–1938)Morris, William (1834–1896)Morris and CompanyMorris dancingmotor tourism, see also chauffeurs, problems of etiquette
That’s just a random sampling, and I’m only up to page 360.