This hint comes courtesy from an anonymous MacOSXHints user and becomes particularly useful when you need to search for specific types of files, while excluding or narrowing the results. For example, the search parameter, love kind:mp3 would yield results of all mp3 files that contain the word "love" in the file name.
Say you need to find a receipt from the Apple Store, but like many of us, you've purchased a truckload of apps from the iTunes Music Store. If you searched for, "apple store receipt," you would get results for any Apple Store receipts as well as all those from the iTunes Music Store. Try using the search parameter apple store receipt -iTunes kind:pdf. This command will show all Apple Store receipts that do not include iTunes and are in PDF format.
These are just a couple examples of how using search parameters can help narrow results and provide more accurate searches. What are some of the search parameters you use in Spotlight to get better results? Let us know in the comments!