If an acorn falls while you're standing under an oak tree, pick it up, turn around three times, and make a wish. Placing the acorn on a windowsill for three days afterwards increases the strength of the wish. This custom probably comes from the old Scandinavian tradition of putting an acorn in the window so that Thor would spare the house.

If you accidentally drop a book, step on it with one foot and make a wish. Be sure to pick it up with the hand that's opposite the foot you used; otherwise, your wish won't come true.

If a candle goes out by itself before it's burned all the way down, quickly make a wish so the spirits can carry it to the wind. This is also supposed to ward off evil spirits.

If you find dew on a morning glory, wet your lips with it and make a wish.

If you lose an earring, make a wish on the remaining one.

If you find a black feather, stick it in the ground so it stands up, and make a wish.
If the feather you find is white, stick it into the crack of a tree trunk and walk around the tree as you make a wish. As long as the feather stays in the tree trunk there's a chance your wish will come true.

A circle of grass that's greener than the rest is called a fairy ring. Stand in the centre, turn around three times, and make a wish. If the fairy ring stays, it can be wished on once every three days.

The version I heard of the 'white horse' wish was to make a wish as soon as you see the horse, then look around for a red-headed woman. If you see one, your wish will come true.
Upon seeing a black horse, one must shake one's fist at it three times and make a wish. If the horse looks at you (possibly because of your strange behaviour) there's a better chance of your wish coming true.
If you see a grey horse, spit first, then make a wish.

If your nose itches, make a small 'x' on the tip and make a wish.
If your ear itches, cover it with one hand and make a wish that only good will be spoken about you.
If your eye itches, refrain from rubbing and instead cover it with one hand as you wish for something or someone you'd like to see.

If you happen to see a jockey who's wearing his riding silks outside the racetrack, make a wish.

If you accidentally drop a knife or any other flatware, make a wish before picking it up.

If you can catch a leaf that's falling from a tree - before it touches the ground - make a wish.

On the evening of your birthday, look at the moon and make a wish. A crescent with its tips pointing up means that the moon may withhold your wish. If the tips point down, your wish will spill forth. A full moon signifies a year of good luck.

If you find two nuts in one shell where there should only be one, share the twin with someone, and you can both make a wish. The first one who remembers to say 'lucky nut' the next morning will get the wish.

A pin found with the point turned toward you means you may make a wish before picking it up.

The purchase of a quilt entitles you to make a wish. Every time you touch the quilt after this, the wish will be reinforced.

When you see a rainbow, make a cross on the ground, and then make a wish.

Any time you sneeze, make a wish. If you sneeze again within three minutes, you'll cancel the wish.

If you stub a toe on your right foot, make a wish. Oddly enough, toes on your left foot don't count.

If you discover that part of your underwear is showing, make a wish and adjust the garment. Failing to do so cancels the wish.

As you eat the first of any fresh vegetable of the season, make a wish.

Words spoken together
If by chance you and someone else say the same word simultaneously, you both should quickly lock little fingers of your right hands and each make a wish. Then each should speak the other's name, to secure the wish.

If you can keep from yawning on seeing someone else yawn, make a silent wish. If you yawn within fifteen minutes, you'll cancel the wish.

When going to the zoo, walk through the entrance backwards while making a wish. When leaving the zoo, remember to walk out of the exit backwards.

A way to ensure success of wishes is to say and do the following: 'touch blue and your wish will come true'.