Make a simple syrup (dissolve 1 cup sugar in 1 cup water over medium heat) and brush it on with a pastry brush or spray it on with a mister onto your cooked cake layers before you ice them. HTH
Handy cake troubleshooting charts:
http://tinyurl.com/2p5bdu
http://tinyurl.com/32goqe
http://tinyurl.com/6c745g
http://tinyurl.com/6lpjww
http://tinyurl.com/yay22w
HTH
This type of question comes up very, very frequently. The best way to get the most thorough answer is to also post what recipe you are using and exactly how you prepare the batter, what temperature do you bake at and for how long.
Really there is no one ingredient you can add to make a cake moist. You can ruin a cake made with sour cream, oil, or any other so called thing to make a cake moist. Prepare the batter the wrong way, or overbake and your cake will be dry.
Simple syrup does not put moisture in a cake. It will help retain moisture already there, but it won't take a dry cake and make it moist.
Post more information and then we can go from there.
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