Regular performance reviews are essential, and the more often the better. Set up meetings every six and 12 months with each staff member to assess their performance and set up new goals for them to achieve.
In these meetings, be direct and concise. Explain what the employee has done well, what they are good at and encourage them to continue. But also point out areas where you believe they can improve. You could also invite them to suggest areas where they believe they could improve.
After this is done, write down a handful of goals you think they can achieve within the six months before your next meeting. Explain what the goal is, how it can be achieved and when you think it can be fulfilled.
If your staff fail to live up to these goals, be vigilant and point out the document you both agreed to. You may need to provide some extra training and education to help them improve, and make sure the goals are realistic, but this sort of benchmark test gives a good indication as to how your staff are fitting in.