Ever wonder why trademarks are so goofy? Most of the time you can't tell what a product is by its trademark name—why would anyone name a breath mint "HailStorm," or "Brexy"? The answer is that a trademark has to be something nobody else would reasonably use for a product name—that way if someone else does use it, you can be sure they're infringing your trademark. Any term that directly describes the product can only become a trademark through extensive use and brand recognition—naming your company's computer "Really Fast Computer" isn't distinctive enough. (And you can never get away with naming a computer "Computer"—the generic term for a product can't be trademarked.) Instead, you have to come up with a name or phrase that gives the impression of speed—"RaceMachine" or "Zippity-Zap" or "QwikZum"—or, even better, a name that has nothing to do with the product ("Echidna Computer Systems"), or a nonsense term that isn't connected to any real word (like "Vactivar" or "Nolotred" or "Xontra").
(By the way, I made up all product names in this column, except where noted. No resemblance is intended to any existing trademark. As they say, all trademarks mentioned here are the property of their respective owners—a tautological statement if ever I heard one.)
In the simplest case, to make a term into a trademark all you have to do is start using it to describe your product. (Preferably after checking to make sure nobody else is using a substantially similar trademark for a substantially similar product.) You don't have to register a trademark; just use a small superscripted "TM" after the first use of the term in any given document, to indicate to the world that you intend to claim the term as a trademark. Registering the trademark with a governmental body does usually give greater legal protection, however. Different nations have different trademark regulations; in the US, trademarks are registered with the Patent and Trademark Office, which gives the trademark owner the right to use the ® symbol after the term. (In HTML, that symbol is denoted by the "®" entity; unfortunately there is no standard entity for TM. Some companies use (TM) after a term; some use a superscripted TM, though that may not appear properly in all browsers.)
Maintaining a trademark requires more than just using it; you have to use it properly. For instance, trademarks should always be used as adjectives modifying generic terms, not as nouns or verbs—"Gargantor(TM) extra-large trash bags hold twice as much trash as regular bags," not just "a Gargantor(TM) holds twice as much trash" or "Gargantor(TM) your trash today!" (In text, you can use the adjective form at first usage, and then get a little more informal on subsequent use—you don't have to say "Venazone(TM) virtual reality simulation software" every time you refer to the product name in the manual.) Trademarks should be capitalized, and shouldn't be turned into plural or possessive forms.
To keep the rights to a trademark, a company must actively prevent others from misusing the mark. If a word comes into common usage as a generic term, it can no longer be claimed as a trademark. My dictionary lists Klaxon(TM) as a trademark, for instance, but I suspect that word has become generic. It also lists Kleenex® as a trademark, which is still true despite widespread generic use. (So next time you ask someone for a Kleenex when you really mean "Do you have any facial tissue?", you may be undermining a trademark. And don't ask someone to Xerox® something when you really mean, "Could you make me a photocopy of that?") Some words, such as "escalator," have definitely become completely generic after starting out as trademarks; the International Trademark Association's "Trademark Basics" article claims that "aspirin" and "cellophane" were similarly once trademarks, but my dictionary gives sensible etymologies (that don't involve trademarks) for both terms.
None of the above explains, to my satisfaction, how PepsiCo managed to trademark the term "Uh-huh!" But perhaps it was only trademarked in a particular distinctive typeface, size, and color... You can establish plenty of things as trademarks, service marks, trade dress, or other forms of intellectual property, but of course any such marks other than words are beyond the scope of this column.
Disclaimer: I am not a lawyer! If you wish to take any of the above information as legal advice, I strongly recommend consulting a legal professional first.
Kleenex® is a registered trademark of Kimberly-Clark Corporation. Xerox® is a registered trademark of Xerox Corporation. Klaxon may be a trademark or registered trademark of some company or other, but I don't know which.