Subject matter experts (SMEs) have expert in their name for a reason. They are your bridge from identified need to successful outcome.  As with most relationships, a foundation of trust and honesty is the best jumping off point for success.

                A great way to begin is to acknowledge the SME’s expertise and skill set.  Be grateful and gracious for their time and be respectful of it.  Build a rapport by discussing their background, professionally and personally.  How did they come to be an expert in the field (Yancey, C., 1996)?

                Once you have built a rapport, clearly state the knowledge gap you are trying to bridge and ask for their opinion on the matter.  Getting buy is key at all points in the process.  You may not use all of the ideas but seeing how the SME views the gap and its root cause will go a long way to building cooperation.

                Finally, share the map you have for developing your Instructional Design and make clear where you will need their assistance along the way.  By setting clear expectations, you show a respect for their time and put them at ease by showing you are a professional and have a plan for success (ACS, 2008).

                Something to keep in mind, SMEs are human and have egos just as you do.  They may be wondering why an outsider was brought in to teach something they are already an expert in (Yancey, C, 1996).  This situation is not insurmountable.  If you find yourself in a situation where there is friction, it is part of your job as an Instructional Design professional to set them at ease. Remind them that while they are the expert on the subject at hand but you are the expert at instructional design and your combined expertise will work together to resolve the identified gap. If there is still friction, you can invite them to pair you with another expert who can serve as a backup to lighten their load or form a team of three.  Sometimes a third point of view can serve as a buffer for strong personalities.  

References

Affiliated Computer Services, Inc. (2008), Working with Subject Matter Experts, Dallas, TX

Yancey, C. (1996). The ABCs of working with SMEs. Performance Instruction, 35(1), 6-9. doi:10.1002/pfi.4170350104