As a rule, always use a hyphen if the meaning of the word or sentence distorts without it, and to avoid duplicating or triplicating vowels or consonants. Hyphenated words avoid ambiguity or join two words together to form a new idea without confusion.
Compound modifiers preceding the modified noun require a hyphen, except when one of the words is "very" or an adverb ending in "ly." Well-known author" is hyphenated, but "very good meal" is not. Suspensive hyphenation is when two hyphenated phrases share a common element, as in "the short- and long-term effects of alcohol," or "10- to 15-year period."