Hyphens are a little like commas in that we often sprinkle them around rather randomly. There are rules to guide you in hyphen usage, though there will still be times when you are left to decide what to do.
Here's a situation in which you do not use a hyphen:
Mark's perfectly coiffed hair contrasted with his lumberjack-style apparel.
You don't hyphenate perfectly coiffed because a hyphen is not used to set off an adverb ending in -ly that modifies an adjective.
Here is another example of that rule:
Senator Lucia Donatelli finally began her eagerly awaited speech.
You do use a hyphen to form compounds such as a once-in-a-lifetime opportunity, a hit-and-run driver, an extra-large pumpkin. You add the hyphen to avoid ambiguity or misunderstanding. (See also lumberjack-style apparel, above.)
Here's another example, in the form of a headline: Bystander helps dog-bite victim. If you left out the hyphen, you would change the meaning of the sentence.
However, if the meaning of a compound phrase is instantly clear, a hyphen may not be needed: a sales tax increase, an acid rain threat.
Hyphenate fractions when used as adjecfives: The audience was one-third teenagers. The vote required a two-thirds majority. You may or may not (your choice) hyphenate fractions when used as nouns: One third of the audience stood to applaud the speaker. If you find it more convenient to hyphenate all fractions, that's not a bad approach.
Hyphenate compounds beginning with self: self-assured, self-confident, self-esteem, self-pity.
Finally, many hyphenated compounds evolve into solid words without hyphens: tonight and tomorrow began as to-night and to-morrow. You may still be struggling with e-mail or email. Style sheets seem to be moving toward the latter, but it's still up to you.
The main thing is to be consistent. If you have a style guide, use it. If you don't, consider developing one that governs both internal and external communications. And have a dictionary handy. Assume nothing. You're not sure? Look it up.