Analyze areas that your choir needs to work on. It's better to have a workshop that's focused on a few specific skills rather than a broad, general meeting that doesn't produce many results. Examples of specific skills might include sight reading, enunciation/diction and vocal health.
Communicate clearly about the time, place and purpose of the workshop. Discuss the upcoming workshop with your members, along with the importance of attending. Post advertisements around the church so your members will be reminded frequently.
Include an incentive for attending the event. For instance, you should provide snacks or a meal. You might even want to consider giving away a door prize.
Consider inviting other clinicians to teach specific skills to your members. The people in your choir probably are very used to your techniques and methods. Having a different person with a new approach might accomplish more in a short period of time than you could accomplish by teaching the skills yourself.
Keep the workshop as brief as possible. You probably will have to host the workshop on a weekend, and most people value their weekend time very highly. Cover your material in a time-efficient manner, and do not go longer than you promised.
Provide handouts pertaining to the skills being covered in the workshop. This allows your members to review at home, and you can reuse the materials together as a review during the next few practices.