We use what are called "Focus Correction Areas". For example, have students write in response to a prompt (can be formal or informal). Then choose 3 and only 3 areas to focus on--one or two can be grammar. For example, perhaps your FCA's for an assignment might be comma usage, subject/verb agreement and strong topic sentences. Have the students then GO BACK TO THEIR OWN WRITING and work on just those things; grade them on just those things as well.
They need to learn grammar in the context of their own writing--ALSO GIVE THEM LOTS OF AMAZING EXAMPLES!!
Also, pick out sentences with beautiful grammar, structure and punctuation and have them do a mimic.
For example: The dog, wet and smelly, rolled on my carpet; however, I loved him, so I forgave him.
Now THEY write one using the exact format:
For example My sister, obnoxious and annoying, barged into my room; however, she was bringing me cookies, so I wasn't mad for long.