Good idea to teach it as a separate subject. One thing I do with my younger (elementary) boys is have them write in a journal every day. I look at what they write, but I do not make any remarks about misspellings, punctuation, etc. in their journals. I just take note of what they need to work on and cover it in a separate English lesson. I'm one of those moms who tends to see all the mistakes jumping out at me on the page and it is hard for me not to say things like, "Okay, you need a comma here, this needs to be capitalized, and this needs to be worded better, etc." By doing it the way I do now, they feel free to write without worry that I'm going to point out all the errors and I sneak in the lesson elsewhere. I do remind them to do their best when they are writing in their journals and to remember the lessons we have been working on. When my kids hit middle school, they tend to take correction better and I can go over their papers in the usual way. I still try to be careful to praise too and not just be critical of their errors. That's been a hard one for me, because I am really fussy about writing.