What are some good tips to help my son with spelling?

My son is 12 and he reads well, but his spelling is terrible. It seems like I've tried everything to help him spell better. What are some tips that you have to help your child with spelling? He doesn't really have any trouble reading. He reads at least 2 grade levels above his grade and his grades are usually good because the reading comprehension grades bring up the poor spelling grades. I'm just concerned that as he goes into Jr. High that his poor spelling will be more of a problem.