How do you know what music to buy? Often, another person tells you – generally in some mediated way. Different media, different people, same principle.

Other than hearing, then liking, the most common and reliable way to find out about music is via the opinion leader. Often this will take the form of a press review or column — or some sort of radio feature. Occasionally, it’ll come from the telly. If you happen to respect the opinion of the person or institution telling you that the record is great, the chances you’ll be persuaded are reasonably high.

You can’t beat the persuasive power of an informed opinion — and this is the reason record labels spend so much time and effort getting their music into the hands of the people whose opinion is respected on these matters.

This is not new information.

But what has changed online is the proliferation of opinion sources. There are internet-only publications with readerships in the tens of thousands, whose writers may wish to say nice things about your music. You must know, for instance, about PopMatters and Pitchfork.

Bloggers, too, are becoming increasingly important. With daily visits and subscriptions in the hundreds of thousands, some of the top blogs need only mention the coolness of something, and that thing’s web traffic will soar through the roof. Look at Boing Boing for instance. They don’t talk about music and records per se — but when they mention something, it becomes the most read about and actively pursued topic on the internet.

More important still are the bloggers who do specifically discuss music. The ones worth paying attention to have developed a level of trust amongst their readers over time. When Jim from Quick Before It Melts says something is good, those of us who read what he has to say will likely be persuaded.

It’s even more interesting when you consider that you’re unlikely ever to see a bad review on an mp3 blog. This does not mean a complete lack of discrimination — quite the reverse. An mp3 blogger will not even bother mentioning something unless it is absolutely, unreservedly recommended. In the music press, the good, the bad and the indifferent all take equal space and the target audience is as broad as it can possibly be. Most mp3 blogs are narrowly focused on the niche, and will only mention the truly great.

So who’s likely to want talk about your music? Well, here’s a list — and another.

But going through a list like that and finding the hundred or so who are likely to intersect with the tastes of those who your music addresses seems a tricky business. Perhaps a better way is to focus on the blogs who are talking about the people who are like what you do.

This is where The Hype Machine comes in handy.

Let’s say your music shares broad areas of similarity with Green Day. A quick search of The Hype Machine reveals a bunch of blogs who have recently posted about Green Day. Those would seem like reasonable people with whom to initiate conversations about your music. Send them a promo. They already nearly like your music and may be inclined to say favourable things to the people who respect their opinions on these matters.

You might even consider becoming an opinion leader yourself. I’ll talk about the power of blogging in more depth later, but for now, just plant that seed in the back of your mind. What better way to surround yourself with an audience of people likely to like your music, than to become a reliable expert on the kinds of music that people who might like your music, might also like?

Blogging’s easy to start, and easy to do — but can I direct your attention towards MOG? It’s an online community — a bit like MySpace, only good — and all about the discussion of music. Becoming an authority in that environment, I’m told, can be a smart way to sell records.

But opinion leaders don’t have to just be knowledgeable individuals. Online, a crowd can be an opinion leader — if it’s the right crowd. One of the most effective ways of upselling online (though it might more accurately be termed ‘cross-selling’) is the Amazon model of ‘People who bought THIS also bought THAT’ (or, as one cynical friend would have it: ‘People who bought crap, also bought shit’).

I know one very smart cookie who bought up a hundred or so of a very popular record on in the same genre as the one he was trying to sell — and each time he did, he also bought a copy of his own label’s record. Before long, the website knew for a fact that people who bought this particular THIS also bought his particular THAT.

Selling the unwanted extra copies at a bit of a loss was a little time consuming, but he factored it in as part of the cost of promotion — and by all accounts, it turned into a successful strategy.

I’m not recommending this to everyone, but it does illustrate a point. Reliable opinions, whether generated by consumer activity like an Amazon cross-sell, or the considered musings of an informed individual, are powerful tools.

The moral of the story is that other than Hear/Like/Buy, a trusted recommendation is the best promotion your music can get — and that sort of recommendation is far more readily available online than off.