Read the story first. As a rule, it's not enough to have just a general idea. You should ideally know all of the details in order to best encapsulate the content in a clear, enticing-to-the-reader manner. Plus, it's important to maintain accuracy with the publication or assignment. You may have an intriguing headline, and the reader may very well finish the story, but do they match up?
Keep it simple. It can be tempting to cram as much information into the title as you can, but remember; that's the reason for the story. All you're trying to do is summarize in as few words as possible. Also, keep in mind that flow is important. Excessive use of colons, semicolons, commas or other punctuation may come across as extraneous.
Engage the reader. As mentioned, the goal is to make one want to learn more. However, keep in mind that you don't have to sensationalize; stick to what the story is about. According to Manchester, UK designer and developer Stuart Brown, keeping the emphasis on key words and hot topics is a good rule of thumb.
Avoid fluff. Getting too crafty for your own good is perhaps the one most ill-advised extreme when it comes to trying to make things interesting or to engage the reader. The content of the story will dictate the tone of your headline. This is where reading the story in full comes in handy. For instance, if you're writing a headline for a story about a dog that is trained to open the fridge and fetch beer, then yes, get creative. However, if the assignment is a hard news piece about a student council resolution on campus, then stick to the basic facts.