I have heard of using coconut, but really doesn't work that well, plus I hate coconut. Any other good tips?

Do you use a crusting icing? If so, you just let the icing crust and you won't have a problem. I've never had icing stick to the cardboard.

And good call on not using the coconut. As I've mentioned so many times that people are sick of hearing it ..... if I get a cake and it has one flake of coconut on it, I would demand a full refund because I would deem the cake totally inedible.

My icing sticks to the cardboard to I used powdered sugar..I read that somewhere and that didn't work to well..maybe I didn't use enough...

What I found that works for me is, I cut a piece of parchment paper the size of the cardboard, lay it flat on the cake, then place my cake on top and refrigerate it for an hour or so. But then again I use Icing that crusts nicely.

Any kind of fine crumbs - corn flakes; graham or other crackers;
Some people use ground nuts but then lots of people have allergies there - as do some to coconut