The principles and methods I'm describing here work across species; they've been used effectively in the training of countless animals, from rabbits to rhinoceroses, from grizzlies to gerenuks. (I highly recommend that all skeptics check out this brief article by Karen Pryor and the embedded video of a marker-trained rhino.) Before her death last year, our lovely calico Hops had learned through marker training to target my hand with her nose and to give me a head butt on the cue "Zidane" (which will only make sense if you follow European football). Most of my experience, however, has been with dogs, and for ease of reference I'll focus my discussion on them.
As I noted in the previous post, it's important that the stimulus you choose as a marker should initially possess little or no meaning for the animal you want to train. This will give you full freedom to endow it with the meaning you want: "Well done! Good things coming!" Here's yet another reason that words can be problematic as markers -- unless you're training a young puppy and you can commit in a disciplined way to reserving your marker word(s) exclusively for training, there's a high likelihood that their meanings will become muddied with unintended associations.
I'm strongly sympathetic to the visceral distaste that many people feel for clickers and other mechanical soundmaking doo-hickeys. They're cold and fussy and seem to require a third hand that we haven't got. Still more annoying, they interpose a barrier of artifice between trainer and trainee. However, for consistency, precision, and repeatability, they're really hard to beat. And paradoxical as it may seem, the little gap they introduce in our "natural" communication with other animals actually improves our mutual understanding immeasurably. No, I take that back -- the improvement can be measured, has been measured, and it's big.
So I recommend a clicker despite the possibility that it may not be totally neutral for you or your dog. Contrary to popular belief, you won't need to use it forever: once you've trained a specific behavior and put it reliably on cue, word markers will usually suffice to maintain the training. But you'll get to that maintenance stage much more quickly with a clicker, and once you're there I can pretty much guarantee that you'll notice a significant uptick in your dog's enthusiasm and attention anytime you pick that silly gadget back up. Likewise your cat's -- Hops would immediately start purring whenever I started a new training session.
Of course, that wasn't her first response to the clicker. She was mostly indifferent to it, maybe a little affronted. The sound it makes is short, sharp, and attention grabbing, qualities that make it effective for training but also make it rude in other contexts. So you'll want to introduce it carefully, from a distance, or muffled in a pocket. (It's best to avoid pointing the clicker like a t.v. remote at your dog's face.) Gauge your dog's response. Curiosity and/or indifference are a fine place to start, but if you see him/her shrink back, you'll need to take things more slowly.
If you decide to use another marker, just do what you can to keep it precise and consistent. A single syllable like "yes" generally works better to this end than a longer word.
Your first and most important task is to persuade your dog that, from this moment on, the following equation holds absolutely true:
"click" (or "yes") = wonderfulness
In order to establish that equation, you need to know what your dog already considers wonderful. Ideally you can identify among the many things your dog loves some thing(s) that are easily doled out in small bits. Food is the obvious candidate for a predatory species, and the one I rely on most heavily (probably too heavily, made complacent by my dogs' eager appetites). For early training, when the strength of a good impression takes precedence over perfect nutrition, I like (because my dogs like) hot dogs and smoked mozzarella, which are relatively inexpensive, have great intensity of flavor, and are easily divvied into tiny (1/4" by 1/4") cubes. Red Barn and Natural Balance also make meaty but dry food rolls that are as mouthwatering (to most dogs) as they are nourishing; they're my treat of choice when I work with dogs at the Oregon Humane Society. But it's good and sometimes necessary to think beyond food rewards; depending on the animal and the situation, they may be impractical and/or unrewarding. I've been working with Pazzo recently on his ability to keep the leash loose when we walk through our local squirrel-infested park (where he may be totally indifferent to food that isn't on the move). When he pulls the leash taut, I simply stop. The moment he gives me slack, I click and move with him in the direction of the squirrel. To Pazzo's delight, the squirrels will often double-down on my reward by staying put, and we've thus become a great slow-motion stalking team. Likewise, agility trainers will often reward their dogs with quick games of tug, and trainers of impassioned herders carefully control their access to sheep.
But for clarity's sake I'll assume that you've got a clicker, a hungry dog, and a stash of small, tasty food treats. Here's what you need to do: