burr bushes (those inch and a half long football shaped, velcro-type seeds) make good cover as the color is very similar and growth speed and size is similar. jimson weed also makes a good color camoflage.
another thing, cut the sharp tips off of the leaves. it makes for a rounder appearance and less pot-like.
growing under and into trees also works well. they don't need direct sunlight all day...a few hours is enough. some have raised buckets in trees and grow the entire IN the canopy...but a bitch to water and care for...