Do not store the books in plastic or use plastic dustcovers unless they are archival quality. Standard plastic covers contain chemicals, which will also eventually deteriorate the paper. One hint also is to find a special pen that you ask the author to sign the book with. It is recommended that you purchase a pen that is also filled with archival quality ink. This will maintain the integrity of the autograph.
Many authors use a permanent marker to sign books.
The pen described above is preferred. When collecting autographed books, of course hard covers are preferred. Do keep the original dust jacket, paper cover on the book also. Books missing this will be rated lower in value. Some older editions may be out of print.