A good way to make the ownership of copyright clear to anyone who buys a painting from you is to do what contemporary artist Karen McConnell does: "I sell most all of my original paintings with a 'Statement of Value' which includes (1) date of sale (2) price paid (3) whether it was purchased framed or unframed and (4) notice that copyright for the work remains with the artist. At the bottom of the form is a place for dated signatures from both myself and the purchaser. I keep a copy, they keep a copy." Another option is to create a certificate of authenticity that includes information about your copyright on it.
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.