Cindy,
I agree, everything needs to be spelled out very specifically. I put the following statement in the agreements that I have customers sign prior to starting, and then again on a handout that I give them about the painting after it has been completed.
"Purchase of this painting does not include purchase of the copyright. The artist retains all copyright unless stated otherwise in writing with the artist signature."
Even if you decide not to charge for copyright usage, you should still control the situation regarding the proof, how many prints etc. Sometimes a customer is excellent and you know there will be future commissions etc. In a case like that you if you decide not to charge them, you might fill out the copyright release form and details, restrictions, state the price, but put "no charge" at the bottom. That way they know what they would have been charged but you are nice enough to not charge them.
It is also clear to them that eventhough there is no charge, there are still restrictions and in the future there might be a charge.
By the way, I looked at your website and really like your portraits.
Mary