Ranting at the off ofs
I'm often asked for ways to improve one's command of the English language. The best I can come up with is to read and to practice using the language. Sorry, no shortcuts.
Reading familiarises you with the way language is used. No need to read anything "useful" -- magazines or novels will do... it's because I've spent so much time reading that I am able to tell whether a sentence sounds right or pinpoint something out of place. I can't explain many grammatical terms or expound on the reasons why a particular sentence structure is wrong, but I know when it's wrong and I know how to correct it.
Unfortunately, nowadays you have to be more careful when reading because the quality of published writing -- not to mention editing -- seems to be going downhill. I first saw the usage of "off of" on blogs (mainly American blogs), and while it irritated me, I figured that it was probably a colloquialism and that the particular bloggers were just writing the way they spoke. But recently I've seen it in magazine articles and certain books -- as in, "I ripped this song off of Celine Dion's latest album" -- and I can't stand it. Atrocious mangling of the English language.
If it's off, it's off. It's never off of. 'Off' already means 'from' (see Merriam-Webster). "Take your feet off the table" is "Take your feet away from the table", and if you were to add another 'of' it would become "Take your feet away from of the table", which is absurd.