The clicker is an often misunderstood device that can be indispensable for training dogs, cats, rats; any animal! It tells your pet when he is doing the right thing and that there's a reward coming in the near future.
A clicker is a small divice, usually plastic and metal, that produces a short, sharp clicking sound. You can purchase them very inexpensively at most pet stores and they are often given away as promotional items by training and boarding facilities.
The clicker is a reward marker. It is not the reward itself, but a message to your pet that a reward is coming. It bridges the gap between the desired behavior and the reward, which means as long as you have perfect timing on the click, you can take your time bringing on the reward. This solves the problem of clumsily dropping the treat or realizing that your treats are on the other side of the room when you catch your pet doing that thing you want him to learn how to do.
I use the clicker only when I'm teaching a new trick. Once a pet learns a trick, the clicker is not necessary anymore. Your pet will know he's done the behavior right and look to you for its reward without the help of a clicker.
Never use the clicker for any other reason. Some people use clicks to get their pet's attention when it's distracted and then followup with a command. This undermines the purpose of the clicker completely. Not only are you giving a reward marker for an undesired behavior (not paying attention), you're teaching your pet to expect a command after a clicker. So instead of staying in that nice down position you worked so hard to teach him, your pet will jump up at the sound of the clicker, ready for action.
Timing is crucial when using the clicker. The click must come as soon the desired behavior happens, not a split second before or after. This can take some practice but you'll get it pretty quickly. I like to hold the clicker in the same hand as either my leash or my target stick if I'm using one. This leaves my other hand free to guide the animal if I need to, to offer physical praise and to reach for treats.
With the clicker, you can train by shaping the behavior, by forcing it, by luring it or simply by marking when your pet offers the behavior spontaneously. In each case, all you do is click the minute the behavior is presented, label the behavior with a word and bring on the rewards.
For example, if you were teaching a dog to sit with the clicker you could do it one of many ways. You could lure the dog into a sit by moving a treat over his head until he sat down, then click the minute his rearend hit's the ground, say "Good sit!" and give him a treat. Or you could follow him around and whenever he sat down on his own, click, say "Good sit!" and then give him a treat. Or you could push his bottom down by holding his front up by the collar and the minute his bottom hits the ground, click and say "Good sit!" and give him a treat. This last is more complicated because it involves using your hands quite a bit, which could interfere with your clicking.
You'll find it helpful to hire a professional trainer to help you refine your technique, even if you only have him or her come for two or three sessions to get your started.
Of course your pet isn't going to magically know what the clicker means when you first bring it out. You have to train your pet to understand that the sound of the clicker means "That's it exactly, now get ready for your reward." He'll catch on quick though, if you follow these simple exercises.
That's it, just click and hand your pet a treat. A few minutes later, do it again. Make sure while you're doing this he's behaving nicely, but you don't need to make him do anything at this point. This exercise is just to associate the click with the treat.
Sit with your pet in front of you where you can get a good view of his eyes. Have some treats nearby and the clicker in your hand. Your pet will wonder why he's sitting there and glance all around. Eventually he'll glance at you. Click right at that second, then give him a treat. Spend 10 minutes or so on this exercise 2-3 times a day for a few days. This exercise not only teaches your pet what the clicker means, it also teaches him to pay close attention to you. After he's got the sit & click down, play this game while taking a stroll, especially if the pet you're training is a dog. It will help later when you teach him to heel.
My Pet Hates/Is Afraid of the Clicker
If your pet doesn't like the clicker, use a different sound. You can use a pen for a soft clicking noise, or make a sound with your body, or select a word, like "Yes!" or "Good" to use instead. Then just use the sound you selected instead of the clicker.
I Keep Loosing the Silly Thing
Use a word like "Yes" or "Good" or a sound you make with your mouth or body instead of the clicker. Then you won't have to worry about losing it.
Practice the "watch me" game a bit more and increase the duration of eye contact to five and then ten seconds before your pet gets a click, then wait five seconds or so before bringing on the reward. Do not use the clicker for anything except a reward marker. At a "non reward marker", that is, a sound that means your pet's made a mistake, like "nope" or "Ooops", say it regretfully, but not forcefully. If you pet pops up after the click, he gets a non-reward marker instead of a reward.
If you're having trouble, consult a pet trainer to help you refine your technique.
A great book that covers the topic of clicker training beautifully is The Everything Dog Training and Tricks Book.