If you're using thick cardboard (maybe to then glue a printed sheet of paper with the art on it) you can also lightly score the fold lines with a knife, so that they will "tear" a bit on the outside and fold back better. But do not cut through fully!
Of course, if you do that and do *not* glue paper on top (or a large A4 label), then those cuts will be visible on the outside, which you generally don't want.