Even with your best blister prevention efforts, sometimes you can't avoid getting painful blisters during a long run. Sometimes blisters are caused by shoes that are tied too tight. You should be able to slide a finger under the knot. If you can't, your shoes are tied too tightly, so loosen the laces in order to give you more room. However, make sure you don't go overboard: Shoes that are too loose can also cause blisters because your foot is sliding around too much, so make sure they still feel snug.