Make it relevant, make it sexy!

Some products/services are sexy, some are not. For all of them you'd better come out with a message relevant to your audience. Then, if you make it to sexy, the job is done!

Here are some tips you might use:
1) Make sure you're addressing the right audience: you must know them, be aware of their specific characteristics and interests.
Tip for Stumbleupon users: SU makes you reach potentially anyone. When you submit your site make sure you use the right tags and description according to point 2, you don't want to hit the wrong audience.
2) The message has to be involving: tailor contents for THAT specific audience. They must be interested in what you have to say. And you must speak directly to them.
3) Make it touching (i.e. "sexy): make them laugh or make them cry and they will do the marketing for you. How? Thumbing up your page, forwarding your link via email, talking about it with friends. In other words: spreading the word!

Now, if you want to sell an expensive business intelligence tool you'll have a harder time than if you had a free videosharing site focused on funny recipes. This doesn't mean tag marketing won't work for you. In both cases it is an highly effective tool. Under a cost/benefit point of view this will always remain a model which is worth the costs. Just make sure you address the right people with the right topics. Then, if you don't win anyone attention, it means no one is interested in you product!