Dear …

I am honored that you would ask me to allow my name to stand for a position on your Executive Board of Directors. __(Name of Organization)__ is certainly a reputable organization that represents a worthy cause. It is with regret, however, that I must decline your invitation. Due to __(time constraints, my schedule simply will not allow me to / a potential conflict of interest because of my involvement with …I cannot/etc.)__ participate on your board at this time. Perhaps I may be able to revisit this down the road if my circumstances change. In the meantime, I ask you to kindly pass along my regrets to everyone.

I hope all goes well for __(Name of Organization)__ this year and that all your efforts prove to be a huge success.

Best regards,