I always add just alittle more vegtable oil to my cakes if pudding is already in the mix - makes them super moist! However, beware of adding too much, it will cause your cakes to fall or worse, cave in under the 5 lbs of buttercream you use to decorate your cake!
Happy Baking. 
I use box mixes and use simple syrup on all of my cakes and everyone LOVES them (which is surprising since they're just box cakes with apple sauce instead of oil--but I'm not complaining!). I just make my syrup with equal portions of sugar and water, warm on the stove until the sugar disolves (becomes clear) and once cooled I put into large mason jars and keep it in the fridge. I use Wilton's cake release and take my cakes out of the pan pretty quickly and wrap immediately with plastic wrap, this helps hold the moisture in as opposed to it all evaporating and leaving the cake dry. Then after I level and before I tort, I use a pastry brush and brush syrup on the entire cake (sides, bottom, top). Then I fill and put into the fridge to let the cake settle before frosting. I have never had a problem with a cake being dry...in fact everyone always comments on how moist it is. One word of warning, do not OVER moisten a chocolate cake. I have had one or two leak on me when I got over zealous with the syrup.
As soon as I take my cakes out of the over I flip them onto a cake coard and then they go in the freezer. They are always super moist.
I have always made really moist boxed cakes, but have recently started adding 1 small box of instant pudding and an extra egg to my cakes (as per another CC'r who is a Wilton Instructor). I have had people tell me they didn't know my cakes could get any better....but they have....and they are denser as well.
Here's the recipe:
1 box of cake mix (any brand, any flavor)
1 small box Jello Instant pudding mix (coordinate flavor with flavor of cake mix)
1 1/4 cup whole milk
1/2 cup vegetable oil
4 eggs
Mix ingredients for 30 seconds on low until incorporated, continue mixing additional 2 minutes on medium.
These measurements work for any brand, flavor of cake mix.
I have been to 2 weddings in the past month that I had done the wedding cakes for, and overheard people commenting that it was the best wedding cake they had ever had....and one was a basic french vanilla cake. I will never go back to making a cake without the addition of pudding.
thank you for your responses, now I know what I have been doing wrong, once I take the cakes out of the oven, I let them cool, before putting in the frig. So you take it out of the oven and immediately put in freezer or frig? then do you defrost (if in freezer) before putting simple syrup. and how much syrup do use on a 8 inch cake?
thanks.
thank you for your responses, now I know what I have been doing wrong, once I take the cakes out of the oven, I let them cool, before putting in the frig. So you take it out of the oven and immediately put in freezer or frig? then do you defrost (if in freezer) before putting simple syrup. and how much syrup do use on a 8 inch cake?
thanks.
I still swear by the wax paper. My cakes are always moist whether I use a box mix or from scratch. I only grease my pans shortening and line them with wax paper. Never had a problem. This week I ran out of wax paper, used paper towels like my sister does, my cakes took longer to bake completely and of course they wear too dry. I use simple syrup on some and another liquid to moisten on others, it was a major DIASTER !! 
wILL NOT do that again.
Also you may want to check your baking time or your oven for the right temperature. You can buy a thermometer, preheat your oven for 10 minutes and place the thermometer in the warm oven. It should read the same as the setting. If its not you can adjust the gage to get the proper temp.
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