I use Duncan Hines all the time and they always come out moist. I bake at 325 - did you over bake the cake?
Or, you could add a box of instant pudding to the batter and an extra egg - makes a wonderfully moist cake. Also, the WASC recipe on this site makes a nice moist cake.
HTH
Jen
I use Pillsbury (that is the kind my distributor sells). I use butter instead of oil, and I bake it at 330 instead of the typical 350 degrees that the directions say. I am trying pudding in the mix with an extra egg in it as we speak (it is in the oven) I will let you know what the results were.
Hope it helps. 
I always use Pilsbury's Pudding in the Mix cake Mix (in a blue box) and bake it @ 325. Always comes out moist. I like it better than any other cake mix (duncan hines or betty crocker)
I've not had a problem with DH. Did you use a heating core (or inverted flower nail to help it bake more evenly?) For a full sheet cake especially. I recently tried this doctored cake mix recipe and everyone loved it: http://www.cakecentral.com/cake_recipe-1972-Durable-Cake-for-3D-and-Wedding-Cakes.html
I'm sure there are many others on this site too, this is the only I've tried since it was so good! HTH
Here is the link to The White Almond Sour Cream cake, better known as WASC. HTH.
http://www.cakecentral.com/cake_recipe-2322-White-Almond-Sour-Cream-Cake.html
I substitute applesauce for oil in all my box receipes and they seem to come out really moist (and have less fat than using oil, yuck!).
Since you already baked the cake, you can try to add some simple syrup to it, it may help. (equal parts of sugar and water, heat on the stove until the sugar disolves and the water becomes clear. I usually cool the syrup, but you can maybe use it warm and brush on the cake, then wrap well with saran wrap.)
You might be able to salvage the cake if it works. Good luck!
Here is the recipe for WASC
http://www.cakecentral.com/cake_recipe-2322-0-White-Almond-Sour-Cream-Cake.html
I use Pillsbury cake mix. I only use 1 cup of water for each box, they always call for more. but I think it makes it too thin, and here's my little secret. In white cake I use cream bouquet, in all others I use butavan, but what I do is I buy the 8oz bottles of those and then I take a 16oz bottle of corn syrup and mix 8oz of cream bouquet or butavan to 8 oz of corn syrup put them back in the corn syrup bottle and then I add one tablespoon of that mixture to each box of cake mix, moist and yummy!
jibbies
Since I found it on this wesite, I have used the extender recipe in every single cake mix....
In addition to the ingredients on the box, add...
one egg
one cup flour
one cup sugar
one 8 ounce container of yogurt OR sour cream
one Tbsp vanilla or other accommodating extract
1/4 tsp salt
Good luck!!!
I made a box mix last weekend, disregarded the instructions and added 4 eggs, 1 c water, 1/3 c oil and 1 box pudding. I didn't use a heating core for my 12" sq. cause I didn't think about it. After I torted and trimed the cake was a little dry. I crumbcoated, froze, masked and when I ate it the next day it was soooo moist and tasty.
I said I would post a reply after my cake cooled. I was the one adding pudding to the Pillbury chocolate cake mix. Well let me tell you...It was great. My dad is not a cake kind of guy, but he loved it, he kept commenting on how moist this cake was. So add the extra egg and a box of pudding...I used chocolate fudge pudding with chocolate cake. Delish as Rachel Ray would say.
I'm glad your cake turned out good. I use DH exclusively and only butter recipe. I've had that same dry cake with DH yellow and I never use it. I also put in an extra egg and a little extra water. I also cook at 325 degrees and about 10 min over time on the box, but it has to be watched.
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