I frequently consult with contact centers, and would add to the following to the thoughtful answers your query has already received...
* Train your coaches to accurately and thoroughly assess agent performance and do lots of calibration to make sure that all coaches are assessing fairly and consistently.
* Establish guidelines related to how often to assess each agent and what coaching method to use in various situations, (e.g., fly-by, two-way conversation, etc.).
* Be sure to inform agents of what's in it for them. If coaching is done well, the agents will welcome it because they'll see that they get lots of praise and are constantly learning and improving.
* Teach coaches to use positive, effective, and motivating feedback models whether they're praising, refining, or correcting performance. (I use the ones at http://bit.ly/9eKx4K
* Hold your coaches accountable for coaching well--and reward them for it. Don't neglect to 'coach the coach'!