There have been a number of questions concerning when to use hypens. The following is an excerpt from The Economist magazine's style guide.
© The Economist Newspaper Limited 2004
Use hyphens for:
1. FRACTIONS (whether nouns or adjectives): two-thirds, four-fifths, one-sixth, etc.
2. MOST WORDS THAT BEGIN with anti, non and neo. Thus anti-aircraft, anti-fascist, anti-submarine (but antibiotic, anticlimax, antidote, antiseptic, antitrust); non-combatant, non-existent, non-payment, non-violent (but nonaligned, nonconformist, nonplussed, nonstop); neo-conservative, neo-liberal (but neoclassicism, neolithic, neologism).
Words beginning Euro should also be hyphenated, except Europhile, Europhobe and Eurosceptic; euro zone and euro area.
Some words that become unmanageably long with the addition of a prefix. Thus under-secretary and inter-governmental. Antidisestablishmentarianism would, however, lose its point if it were hyphenated.
A sum followed by the word worth also needs a hyphen. Thus $25m-worth of goods.
3. SOME TITLES
vice-president, director-general, under-secretary, secretary-general, attorney-general, lieutenant-colonel, major-general, field-marshal
general secretary, deputy secretary, deputy director, district attorney
4. TO AVOID AMBIGUITIES
a little-used car
a little used-car
fine-tooth comb (most people do not comb their teeth)
third world war