One of the best ways to increase casting distance in Stardew Valley is to level up your fishing skill. As you level up, you will unlock new fishing rods that can cast farther. Additionally, upgrading your fishing rod at Willy's shop can also help increase your casting distance. Another way to improve your casting distance is to use bait or tackle that specifically boosts your casting range. Finally, timing your cast correctly and using the right amount of force can also help increase your casting distance.