When you train your horse to do a trick, even the simplest one, you're doing two things:

1.) You're opening the lines of communication between you and your horse. Your horse learns that by paying attention to you, something is gained. That can be either a treat or praise. In this way, your horse becomes more responsive to your other commands as well, and pays better attention to you.

2.) You're teaching your horse to learn. With each trick, it becomes easier for your horse to learn new tricks. You learn many different things in school but it all started with learning the alphabet and counting to ten. The more you learn, the easier it becomes to learn more difficult things. 

It's been proven that teaching colts tricks at an early age makes them easier to train than colts that weren't taught any tricks at all.

1.) Keep it short. Horses have short attention spans. Don't take the fun out of learning by tiring your horse with long lessons. Ten to fifteen minutes is plenty of time to work on something new. 

2.) Be consistent. Very short lessons each and every day are all that's needed. Don't skip a day if you can help it. Short lessons each day give your horse time to think in between. It may seem like he doesn't understand anything at first. He'll use the time between lessons to reflect on what happened and that's often when he'll suddenly "get it."

3.) Make the reward worth it. If you're using treats as a reward, make sure your horse is hungry. Treats right after a meal won't be nearly as rewarding as treats a long time after a meal or right before a meal, when your horse is really hungry. If you're using praise, a horse that just spent the last few hours alone will be more responsive to a kind word and stroking than a horse that you've just brushed, bathed, or spent some time with.

4.) Be patient. Some horses learn quickly, others slowly. A horse that's learned one or more tricks will begin to learn new tricks much faster than a horse that's done nothing but carry riders his whole life. A trail horse will be accustomed to ignoring anything you say or do that has nothing to do with riding and will probably ignore vocal commands at first.

5.) Tiny increments. Everything we do with horses is done in tiny increments. You don't teach your horse to bring his bucket in one day! 

First he has to acknowledge the bucket.  Then he has to:

1. touch it. 
2. put his mouth on it. 
3. grip it in his teeth.
4. pick it up. 
5. learn to keep it in his mouth and not drop it. 
6. learn to walk toward you with it in his mouth.
7. learn to hold it until you take it from him.

Right before feeding time is an excellent time to spend the few minutes needed to work on a new trick.

For example:

1.) Many horses will stomp their feet when impatient. You can teach your horse to shake hands by tying him and waiting for him to become impatient. When he raises his foot to stomp it, grab it and hold it saying "Shake" and give him a small treat or praise.

Stand to one side or the other so you don't get a hoof in the shin as he stomps his foot.

This will shock your horse. He was actually rewarded for something he's usually chastised for. You'll almost be able to see the wheels turning in his head. After his surprise wears off, he's certain to stomp again. Again, he'll be rewarded.

After several times, you can begin to put out your hand and say "Shake" before he stomps. Don't let the lesson go on too long. If he raises his foot after you put out your hand and say "Shake", the lesson is over. Praise him exhuberantly. Act like he's the smartest horse you've ever seen and make him feel like a king. 

If he doesn't "get it" enough to raise his foot when you put out your hand and say "Shake", quit after ten or fifteen minutes. The lesson is over even if he didn't offer his foot to your hand. 

This wasn't a waste of time. During the time between this lesson and the next, he'll think about what happened. It might take several lessons before he makes the connection between his action and the reward. Be patient and don't push. Keep the lessons light and fun.

Once your horse offers his foot to you when you put out your hand and say "Shake", you no longer give him a treat for lifting his foot and stomping when you haven't asked him to. It's very important not to reward your horse for tricks unless you've asked him to do the trick. He must know that he's only rewarded for responding to you.

The next step after he's learned to offer his foot is to stand farther and farther away so that he has to put his foot out forward instead of just lifting it as he does when stomping. Any small improvement over the previous lesson, just an inch of forward movement, should signal the end of the lesson immediately and the large reward.

One side effect of using a bad habit to teach a trick is that it usually cures your horse of that habit. Now, instead of stomping, he'll want to raise his foot to "Shake hands" in hopes of a reward, since stomping gets no reward. Sometimes the bad habit actually loses its appeal when converted into a trick this way.

I don't like to teach a really large horse to shake hands when it will be around small children. The horse may only want to "shake hands" and end up hurting a child by accidentally kicking them or knocking them down. Also, always stand to the side when performing and teaching this trick to prevent yourself from being injured as well.

2.) A common expression of a horse that wants something is to shake his head up and down. Maybe he's hungry and wants his grain, maybe he's thirsty and wants water. Maybe he just has a habit of making this motion whenever you come around thinking it will get him grain or attention.

You can use this expression to teach your horse to shake his head "yes" in exactly the same way you used stomping to teach your horse to shake.

Begin by waiting for him to make the motion, say "Yes" or something similar and shake your head also, giving him a treat. Slowly work you way toward getting him to respond after your commands instead of before them, and you're on your way.
 

Most horses aren't used to listening to the words that come out of our mouths. This has to be learned and it takes time. Hand signals should be very specific and can always be dropped later if your horse seems to catch on to the spoken word.
 

1.) Instead of hand feeding treats, place them in a bucket or on the ground, so your horse doesn't associate your hands with food.

2.) When your horse attempts to nip, place your fingernail under his chin and push up. This is unpleasant. When you do this properly, from underneath, your horse won't associate the unpleasantness with you or your hand, but with the behavior instead, and will stop the behavior without becoming "hand shy".
 

It only takes ten to fifteen minutes once or twice a day and patience is the key. Do everything in tiny increments and reward your horse enthusiastically.

Keep it short and above all, have fun.