It seems that most of the sheet cakes that I make end up being dry. I use flower nails and cake strips so it cooks evenly. I usually cook until golden brown, pulls away from the side and cake tester comes out clean. Why is it so dry?
if you're baking till the cake pulls away from the side then it's already overcooked. Bake it just till the center test done. Overbaking will cause it to be dry.
Yep I agree with Leily, if it has pulled way from the sides, it is already in the over baked side of things.
Your cake tester also doesn't have to be completely clean. A few crumbs on it is fine, as long as it isn't gummy.
If you are stuck with a dry cake you can moisten it up by wrapping the sides with damp paper towels and wrapping it in plastic wrap or a ziplock for a few hours then frost/fondant immediately. Yes never wait 'til the cake pulls away from sides.