A short note (handwritten is particularly good) thanking them for a good job is extremely powerful. Particularly for new employees on your team. Or, for employees new to the workforce and early in their careers. Include in your note a sentence regarding what they did especially well and how their specific action made a positive impact. Remember, be as specific as possible in what you write.
Be sure to send your note soon after the job was completed. If you wait too long (more than a week), the note will lose its impact.
Send your note in a way it can be easily saved by your employee. Even employees who have been on your team for a long time will likely save your note. I still have 20-year-old memorable thank you notes in a file.
Finally, reserve your sending thank you notes for the big jobs, large projects, extra special work. If you send thank you notes too often they'll lose their impact.
