water regularly, and thinning out does it for mine. i thin them out to about 1/2 inch apart when they are strong seedlings but havent yet formed their first true leaf. and then again if any of them look particularly squashed later on. also, when i harvest, i dont always pull the very biggest, but sometimes go for the slightly smaller radish a few spaces down as pulling that will give more space to two adjoining ones (if that makes sense).
between the flea beetle, and the baby slugs who also love my radishes, i grow enough to get sick of them, i think :D
