iVillage

If you see a piebald horse (one that is spotted and more than one color), spit over your right shoulder, close your eyes, and wish. If you see the horse's tail, your wish won't come true.
(England)

A spotted horse is considered to be particularly magical; if you see one, close your eyes to make your wish, then look back at the spotted horse again and say "so be it."

If you see a black horse, shake your fist at the horse three times while making a wish. If the horse looks at you, there's a better chance of the wish coming true.

If you see three black horses, shake someone's hand. (Shake your own if you're alone.) Make a wish and look away. If you see the same horses again, your wish won't work.

If you see a white horse, cross your fingers and make a wish.

Some say if you see one white horse you will have bad luck if you don't cross your fingers or stamp your foot once and then make a wish.

If you see a white horse in the street, make a wish, but don't turn or look back until you're sure the horse is gone.
(England)

If you see a white horse, make a wish, then look around for a girl or woman with red hair. If you see one, your wish will come true.

If you see a person with red hair, look for a white horse. If you see one you are probably a lucky person.

When you see a white horse, make a wish, lick your thumb, and press it into the palm of your other hand. Stamp the wet palm with your fist while saying:

"Crisscross, white horse."
If you stamp "crisscross, white horse" one
hundred times, your wish should
come true in a month's time.

(In some places the chant is:
"Crisscross, white hoss,
Money for the week's done.")

When you see a white horse, make a wish and say:

."White horse, white horse, ding, ding, ding.
.Grant my wish for a very good thing."

(In some places the chant is: