Be sure to own your cover files and all art and creative rights… not just copyright.. but actual rights to the work done. Be sure the designer you use is working as work-for-hire…therefore you own the rights to the artwork created. Most designers assign the rights on final payment but some might keep the rights whether you’ve paid or not… be sure to get it straight in the beginning.
ALSO, just as important is being sure you receive all the files that were used to create your cover, not just a final pdf. You need the source layout files, image and photoshop psd layered files if any, and eps.
Be sure that you include you owning the rights and receiving all the files, in any agreement or contract you sign.