Forming plural compound nouns

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When a compound noun is expressed as a single word, pluralize the final element in the compound as if it stood alone. (Basic rules for forming plurals)

printout / printouts
photocopy / photocopies
forefoot / forefeet
toothbrush / toothbrushes
workman / workmen

The plural of hyphenated or spaced compound nouns are formed by pluralizing the chief element of the compound.

father-in-law / fathers-in-law
looker-on / lookers-on
attorney at law / attorneys at law
couch potato / couch potatoes
runner-up / runners-up

BUT
passerby / passersby

When a hyphenated compound does not contain a noun as one of its elements, pluralize the final element.

get-together / get-togethers
hang-up / hang-ups
run-through / run-throughs
know-it-all / know-it-alls
hand-me-down / hand-me-downs
drive-in / drive-ins

The plural of compounds ending in ful are formed by adding an s.

armful / armfuls
cupful / cupfuls
pocketful / pocketfuls
teaspoonful / teaspoonfuls

When the first element of a compound is a possessive, simply pluralize the final element.

collector's item / collector's items
seller's market / seller's markets
women's service / women's services
visitor's permit / visitor's permits