The best way to moisten dry muffins is to keep them from getting dry to begin with. To do this, don't over-mix the muffin batter and don't over-bake them in the oven. If it is too late and your muffins are dry and crumbly, the best thing to do is to create a simple syrup and brush all the surfaces of the muffins with the syrup. Simply bring equal parts of granulated sugar and water to a boil over medium heat then remove from the stove. Use a pastry brush to cover the dry muffin with the simple syrup mixture. Let sit for 15 minutes and repeat with more simple syrup if needed.