Just sent them a kind card of thanks for their relationship with your business, and add a note that having a time frame to plan ahead with would be most helpful to you in regards to scheduling future efforts. Ongoing communication is good to have, so that when questions come up, there has already been contact, making things less spotty. Perhaps you could use periodic updates, something to keep you on their mind. Contact doesn’t have to be frequent, but regular is important. If they come to expect to hear from you from time to time, there will be no awkwardness about approaching new subjects.