Avoid using cabbage, spinach, broccoli, brussle sprouts, or any other cruciferous vegetables. The are calcium-binders which prohibit the absorption of calcium.
I agree with Marcia about the poor calcium binding with these veggies. That said, I do occasionally give my feeders (and my beardie) veggies in this category. In the case of the bearded dragon, it's for variety and she gets plenty of other non-problematic veggies and in the case of the feeders, I'm going to be dusting them with calcium anyway. The simplest thing is probably not to use these veggies since there are so many other choices. My point is that if for some reason you end up occasionally using them (e.g. your feeders need moisture and that's the only thing around, or someone who's taking care of stuff for you does it) it's not the end of the world.