This is the single most important thing to remember. Common interest, by choice. Treat each newsgroup as a separate neighborhood, and you'll do fine. The way to do this properly is to pick your groups wisely, and wait for someone to ask about something related to your field of business. For some people this will be easy.
Analogies are fine, as long as they are clear and relevant.
Trying to make your business fit where the fit isn't obvious will be poorly received. Any time you refer to your business, make sure that you are providing information that is useful even if the reader of the post chooses to use someone else's business, or none at all. The object of the indirect ad is to show that you know your business and are the type of person that someone would want to do business with. You accomplish this by being knowledgeable and helpful. The post is your ad.