Inkjet photo and fine art paper on a roll will almost always have a pronounced curl. The heavier and thicker the paper, the more curl you will encounter. Curl will increase as you get closer to the roll's core.
It is helpful to understand why paper curls and stays curled. Inkjet paper is usually made from tree or cotton rag fibers. These fibers are fairly short and are aligned during the paper making process so that they run the same general direction. When put onto rolls, paper is grain long - the fibers are aligned along the length of the roll. When curled around a core, this setup allows paper to gain a "memory" that is persistent after a short period of time.
To truly de-curl paper, you need to "break" the curl. There are a few ways you can accomplish this to some degree. Please keep in mind that curl is persistent. Extreme measures, see option 3, are usually required to truly flatten the paper.
Option 1 - Flatten the print under a heavy weight (for light curl)
This is the easiest and yet possibly least effective method. Lay your print on a large flat surface and cover it with a weight. The weight should be evenly distributed across the print. If possible, place protective paper on top of the print before applying weight. You will want to leave the weight for as long as a week for maximum effectiveness.
Option 2 - Reverse wind your print
This is the recommended option given the effectiveness of option 1 and cost of option 3. Take your print and lay it on a large, flat surface print side up. Carefully roll the print up so the image is on the inside and not visible. Pay close attention as dents and dimples can occur. Secure the print with 3-4 wide rubber bands placed at even intervals. Leave the print for 24-48 hours. A rule of thumb is that the thicker the paper is, the longer you wait.
Option 3 - Use the De-Roller
The De-Roller is a device specifically made to remove curl from inkjet prints. It comes in 24" and 50" widths. At $240 to $300, the De-Roller is an investment. From the manufacturer's website:
"The Bienfang De-Roller will allow you to take the curl out of both photo and fine art papers that come off a roll. Most papers can be "De-Rolled" in just a few seconds. Non rag papers will require more De-Rolling time than 100% rag papers. This means more time rolled up in the De-Roller, but rarely more than a few minutes. Note that the De-Roller is also useful for removing wrinkles from documents. This is useful when a document needs to be scanned, allowing for the cleanest digitization possible."
Option 4 - Store boxes of paper upside down
If you notice paper curling up on the ends in the box, store the box upside down so that the weight of the paper is constantly pushing opposite the curl.
Option 5 - Dry mounting curled papers tip from a customer
"When dry mounting fiber based papers years ago, we always kept around rejected prints to dry mount to the reverse side of the mounting board. This precisely counteracted the curl that occurred as paper bases expanded and contracted with changes in humidity. My guess is this would be a good practice with inkjet papers as well."