The main reason to use a hyphen is to ensure that your meaning is clear. Your guide to deciding whether to use a hyphen should be whether the phrase or word is clear without it:
Hot water bottle [could mean a water bottle that happens to be hot]
Hot-water bottle [means a specific item – a bottle for hot water]
Most successful students [could mean the majority of the students who have done well]
Most-successful students [means the students who performed the best]
Reading the sentence aloud can help you to decide – if you naturally tend to run 2 of the words together to make the meaning clear, you might need a hyphen (or the word might be set solid to form a new combined word).
