Immediately upon taking them out of the oven, use two spoons or the edge of your spatula to push the edges of the cookies toward the center. See this Chewy Chocolate Chip Cookies post
for more details. Do this right away, within 60 seconds of taking them out of the oven, before the edges have a time to crisp. This is what makes the center of your cookies ultra chewy and fudgy! It's my favorite cookie tip ever.