I do quite a lot of writing, be it for my job, for this blog, or for other reasons. When writing, the single toughest part of the task is invariably the introduction, which requires breaking the blank page that previously existed. Fortunately, I have developed a useful process for putting together good, workmanlike introductions to complete this task, and allow the rest of the text to be assembled. Thus, for general reference purposes, here is my guide to writing an introduction.
The primary purpose of an introduction is to set the scene. Whatever the topic is going to be, the writer has to assume that the reader is coming to it 'cold', and so needs to be eased into the subject before he or she is ready for your primary argument. Therefore, your goal with the introduction is two-fold: introduce your subject, and then say what you're going to say in the rest of your document.
Additionally, it is important to note that your introduction is nto the document itself. It has a job to do, it should do it, and then it should step aside to let you get to the 'good stuff'. After all, if you're having so much difficulty with your introduction, why should your reader suffer too? A good introduction, then, should be all of three or four short sentences. It should say what it must, and then stop.
In your first sentence, then, you should introduce the overarching subject under discussion: at work, this generally means saying something about the specific project. Here, the subject is 'writing'. You don't even need to say much, or indeed say anything of substance. Just put the broad subject front and centre in your reader's mind.
In your second sentence, you want to drill down to the specific subject under discussion. So, if you're writing a design specification for some component, say something about the component in question. If you're writing a rebuttal to another piece, refer to that piece. And if you're writing a how-to guide, describe the problem that you're writing about.
And then, in your third and final sentence, give the briefest of synopses of what you're going to say. If it's a design specification, now is the time to say that it's a design specification. If it's an argumentative piece, outline your basic argument (don't bother with evidence, just say what it is). And if it's a how-to guide, say that it's a how-to guide.
(If you look at the first paragraph above, you'll see this same pattern - the first sentence gives the broad topic, 'writing'; the second gives the specific problem, 'introductions'; and the third and fourth sentences indicate that this will be a how-to guide giving a solution.)
And that's it. Once you've written that you have an introduction. Now go and write the rest of your document!
Fix it in the Redraft
One thing that's very important to note is that the process above will give you an introduction, one that will do the job. But I've said absolutely nothing about writing a good introduction.
As I noted above, the primary purpose of the introduction is to set the scene. However, a good introduction has a secondary purpose, which is to grab the reader's attention, to make them want to read the rest of the document. Following a simple formula, as described here, gives no guarantee that this will be achieved.
I don't really have any advice how to turn a functional introduction into a good introduction. But I do have one very firm piece of advice: fix it in the redraft.
The thing is, when you sit down to write your document, it's very likely that you will know most of what you want to say. The things that you won't know are the introduction and the conclusion. That's why introductions are so agonising - they really have to be written first, you know what you want to say... but you just can't find the best words to say it.
So cheat. Write a crappy formula-introduction first (and fast). Then write the rest of yuor document - the bits that you know what you're saying. And then go back and redo the introduction. (This now even has the advantage that you know exactly what your document has said, so your introduction can now speak with authority. When you first write it, your introduction runs the risk that the document may end up somewhere you didn't intend.)
Just don't spend an age worrying about getting the introduction exactly right before you write anything else. That way madness lies!
(Incidentally, here's a secret for public speaking - unless it would be bloody stupid to do so, the very first things you should say are, "Hi, I'm {name}, and I'm here to talk to you about {subject}." Because unless you're spectacularly nervous, you're not going to get your name wrong, and simply getting that first sentence out will help settle your nerves for the second sentence. It's quite astonishing just how well it works.)
And now... tell me all the ways the above is wrong. In 500 words or less. Go!