I find that most people run in to hyphenation issues when it comes to compound adjectives, so let's take a look at that.
- A conference held at a high level becomes a high-level conference.
- A building ten stories high becomes a ten-story building.
- A PC that delivers a high level of performance, carries a low cost, and is easy to use becomes a high-performance, low-cost, easy-to-use PC.
When these expressions occur somewhere else in the sentence, occur in a normal order, and in a normal form, drop the hyphen. If they occur somewhere else in the sentence but are in an inverted word order or altered form, keep the hyphen.
The list goes on and on with compound words... adjective+noun, compound with a number or letter, etc. As a general rule, if the compound occurs before the noun, you'll hyphenate it. There are some compounds that don't need hyphens, like real estate and high school, because they are well-established and easily recognized.
When in doubt, reference a dictionary. If you don't find it listed, use the patterns outlined above. And if all else fails, you know where to find me... I'll help you out as best I can!