There is such a thing as Standard English, and as long as that standard is sensibly defined, it is useful in many situations. The problem, though, is that there are many different notions about what constitutes Standard English.
So if you feel that writing in Standard English is a worthwhile goal, I won't complain. If you want to help others towards that same end, good for you. But as you do so, keep the following guidelines in mind:
Find a defensible standard
You are entitled to adopt any standard for "correctness" that you want for your own writing. If you want to write like an English gentleman from the beginning of the twentieth century, that's your prerogative. In that case, Fowler's might be the usage book for you. Heck, you can even write like Chaucer if you want. If you're going to criticize people for their usage, though, you need a defensible set of principles to justify imposing your own notions of correctness on other people. "Because I said so," or "because it says so in [insert favorite usage guide here]" is not enough. The only real authority as to what constitutes Standard English is the usage of actual writers.
"Which writers?" you ask. I'll let you pick the set as long as it includes a reasonably broad cross-section of different authors, some of whom are contemporary. In other words, if there's some particular author whose usage appalls you, I'm happy to exclude them from the canon. (NB: yes, I deliberately used them with a singular antecedent. The horror! If it's good enough for the King James Bible, it's good enough for me.)
Keep your assumptions realistic
Make sure your assumptions about language bear some resemblance to the facts. For example, language changes. When we stop to think about it, of course, that is trivially obvious. When that change happened before our time, we have no problem coming to terms with it. We're all very comfortable with the fact that the language we speak is different from that of Shakespeare's time, and we don't think of our language as somehow inferior to that which people spoke 400 years ago—just different. But somehow, we hit the roof when we see the meanings of words changing around us, as if somehow language should have stopped changing after we mastered it. Words mean what the community that uses the word think they mean, not what some self-declared elite say they mean. If enough people reinterpret the word's meaning, the meaning changes, the shrill protests of usage guides notwithstanding.
I can hear your indignant questions now: does that mean every odd usage by anyone and everyone counts as Standard English? No, of course not. It does mean, though, when many writers and speakers of Standard English adopt a particular meaning of a word, it's time to concede the point. For example, the word hopefully has been used to mean "it is to be hoped" for about a century. By now, that's an ordinary meaning of the word. For the overwhelming majority of people, there's no problem with it. To claim that hopefully must always mean "in a hopeful manner" is simply fatuous. It's just as ridiculous as insiting that a computer can only refer to a person who calculates, since that was the earlier meaning of the word. It contradicts the facts about actual language usage. Strunk and White contains many soi-disant misused words whose meanings have already changed for most users of the language.
A good place to start educating yourself about the realities of language is Language Myths by Laurie Bauer and David Trudgill.
Look it up!
There is a lot of misinformation floating around about what constitutes Standard English. Before you blast someone for splitting an infinitive or ending a sentence with a preposition, do yourself a favor and check what you're asserting.
The same warning, by the way, applies to any story you may have heard about word origins. If someone tells you that "rule of thumb" comes from an eighteenth-century law that men could beat their wives with a stick no thicker than their thumb (the usual moral of this story is that the expression should therefore be avoided as offensive), go check the Oxford English Dictionary. You'll discover that the story is utter bunk. As a rule of thumb >cough<, the more elaborate the story behind the word, the more likely it is to be false.
Know the variations
Different guides give different recommendations about various points of usage. Pick something that's congenial to you for your own writing, but just because the guide you like insists on one way does not mean that other variations are wrong or that you are entitled to demand others to do it your way. If you want a good reference work that will explain the conflicts, you definitely should get a copy of Merriam-Webster Dictionary of English Usage.
Don't confuse house style with correct English
References such as the Chicago Manual of Style, or the APA Publication Manual dictate a level of uniformity in usage that is much greater than Standard English actually requires. Thus, for example, APA style insists on that rather than which for integrated relative clauses (e.g., "The problem which confronts us cannot be resolved by good will alone."). To publish in a journal that follows the APA standard will require you to follow that usage, but a cursory glance at scores of canonical authors will show that which is just as standard as that in such cases.
Don't confuse formality with correctness
Standard English is not the same as formal English, and to confuse the two muddles the difference between a grammatical standard and stylistic register. And once we introduce register into the discussion, it's easy to see that the same level of formality is not appropriate for every situation. To insist that one must always say, "It is I" is asinine. Just imagine calling your significant other on the phone and saying, "Hello, honey, it is I." Would anyone really be such a pompous ass? And if no one is actually speaking that way, where do we get off saying that "it's me" is incorrect? See this article by Peter Trudgill for a discussion of what constitutes Standard English from a British perspective.
Take responsibility for your judgments
If you must use your notion of correctness as a club to wield over others, at least have the intellectual honesty to admit that's what you are doing. You often run across people trying to justify the enforcement of prescriptive rules by an appeal to the negative judgment of others: "If you don't do X, you will be judged to be uneducated."
Don't distance yourself from the judgment by taking refuge in the passive voice. You're making the judgment, not some unnamed actor. More often than not, the unstated judges simply won't notice. There are, it is true, some things that will be widely noticed and that serve as legitimate social markers dividing Standard English from other varieties. That set of items is far smaller than you might think, though.
In his classic article, "The Phenomenology of Error," Joseph Williams provides a useful way to think about which rules really count and which rules are fluff.
Don't romanticize the past
There never was a golden age when every school child learned and followed all the schoolbook rules that passed for grammar. There is no evidence that those who did follow the rules were better writers because of it. If you say to me that it's a shame few children today learn formal grammar, I will agree with you, but returning to those old schoolbooks, with their muddled, misleading, and in some cases outright incorrect statements about the English language does no one any good. We need grammar books that make defensible, factually supported assertions about English, not reprints of old warhorses like Warriner's.
You know much less about language than you think you do. (I don't exempt myself from this dictum, by the way, as paradoxical as that seems. I'm constantly discovering that something about which I was sure wasn't so.) If, for example, you think that some odd feature of language is an innovation, it's probably been around a lot longer than you thought. The more dogmatic you are, the more likely you are to be struck by The Law of Prescriptive Retaliation.
If you must go prescriptive, don't go postal. Some people start to froth at the mouth whenever they start to think of the errors that really bother them. At best, it's unseemly. Departures from Standard English are not departures from morality. You are not a more virtuous person because you have mastered a set of arbitrary conventions. Branding someone as ignorant or slothful because they don't use English the way you want them to is an ad hominem attack. And ask yourself what you are hoping to achieve from your rant anyway? Do you really think that the offending person will be more likely to change their behavior if you shout louder or become more insulting? That hardly seems plausible. Are you trying to establish your own superiority? Perhaps, although I'll do you the courtesy of assuming that's not a conscious motive. Maybe you're actually ranting for the benefit of others, trying to persuade them to adopt your usage in order to avoid suffering the same abusive fate as your victim. If so, that's an ugly rhetorical trick, one which, if your basic argument is sound, you shouldn't need.