Always select one important professional development skill or competence that your staff member needs to develop or improve.
To identify that goal, look at the work that needs to be done. Perhaps the person needs to sharpen a technical skill, develop a strategic skill, handle conflict better or communicate more effectively. Talk with your staff member and have them create a short write-up of what the goal is, why it's valuable and what steps they can take to achieve it.
Then - and here is the part that makes a big difference but that most people skip - look for work opportunities where the person can apply this goal. To cap the effort, check in every other month for the next several months to discuss progress toward the goal.
Sometimes take the opportunity to offer real-time feedback about an area of professional development in their current role. This is not something you will the have time to do weekly, but keeping an eye out for learning opportunities can make a difference. Providing real-time actionable feedback will go a long way toward meeting your goal of positive change.
Additionally, take time once a quarter to conduct group professional development exercises with your whole team. Everyone will get something out of it without anyone feeling singled out. Furthermore, it's a tremendous way to promote team camaraderie.
Never ignore it. Don't let too much time pass without doing some form of professional development for your people. This is one of the cases where you are making an investment in people. We hear a lot about doing that, but turn the words into actions.
Read more about mentoring in CIO's Mentoring Drilldown.