Taking these into account may make stories a bit more authentic, as they are things that people like me, who are outsiders, wouldn't generally know about. Some of these may be truth, but most of these seem to be simple superstitions.
I wanted to make a list of all the ones I knew of, and ask for the contribution of those who are familiar with the Japanese society, and particularly of those Japanese people in the group. If you find something that is not actually believed nowadays, is fake, or you know of one that isn't on the list, I'd be grateful for your help. :)
Edit: added a couple more that I just remembered about!
If you sneeze, it’s because somebody is talking about you.
If you sleep on the floor, you’re going to catch a cold.
If you sleep with your tummy exposed, or spend a lot of time with your tummy exposed, you’re going to get diarrhea.
If you soak for too long in the bath you’re going to pass out from the heat (this really happens).
If you fall asleep on your desk while chewing on a pencil, you may die (because you’d be likely to fall on the pencil and it’d sink in your flesh, possibly reaching the brain, or something like that).
It’s not very manly to like to eat sweet things.
If you are a man and repress your sexual impulses when you’re young (generally meaning, if you don’t jerk off often), you’re going to be a pervert when you’re old.
Tea should be avoided when you’re on a diet (despite the fact that they generally drink it unsweetened).
Showing bone is dirty, so you shouldn’t show your teeth when you smile (not very much in use nowadays).
Wearing a fundoshi is very manly (even if you’re a skinny guy, it would seem).
Sumo is the manliest, most sacred of sports.
Shinto maidens are very pretty.
Buddhist nuns are very pretty (even with the shaved head).
Having a shaved head/very, very short hair is very manly (alternatively: makes you look like a monk/athlete. Depending on who is saying this, this may be a good or a bad thing).
People with bleached hair or no eyebrows are punks.
If you suffer a heartbreak, you should get a haircut (to start anew, I suppose).
School uniforms can be used as formal wear; i.e.: can be worn to weddings, funerals and ceremonies.
Wearing a suit and shades (especially at night) makes you look like a yakuza (despite the fact that yakuza nowadays dress pretty much like everyone else).
You should never open a charm (the kind you can get at a shrine or a temple), or it won’t work anymore.
You shouldn’t clip your nails or toenails at night, or you won’t make it to your parent’s funeral.
You shouldn’t go to sleep immediately after eating, or you’ll become a cow (!).
When you’re walking past a cementery or if a funeral hearse drives past you, hide your thumbs, or something bad will happen to your parents.
Whistling at night will attract snakes/spirits/thieves.
Wearing bells keeps bears/spirits away.
Four is an unlucky number because it sounds exactly the same as “death” in Japanese. Nine is also unlucky, because it sounds like "pain" or "suffering".
Nine is also unlucky, because it sounds like "pain" or "suffering".
You shouldn’t walk through small spaces (like the space between a lampost and a wall) because they’re a gateway to the spirit world/you’ll get spirits stuck to you/it’s generally unlucky.
When apologising to somebody, you should bow (it’s not strange to see people bowing while apologising on the phone...).
Looking people in the eye when you talk to them is rude.
A woman who is giving birth/people in pain in general shouldn’t cry, moan or scream so as not to bother other people. Men who complain of pain aren’t manly.
When somebody returns from a funeral, they should have salt thrown to their back before stepping into the house so as not to bring spirits home (not very much in use nowadays).
Those who interfere with love shall be kicked by a horse and die (though on the absence of horses, it would seem the last part is being replaced by "will be punished" or similar these days).
When their baby teeth fall down, Japanese children should throw the lower baby teeth up onto the roof, while the upper teeth are thrown under the "en-no-shita" (the area below the floor of a traditional Japanese house), so that the upper teeth will grow healthy downwards and the lower teeth upwards (though it seems more and more children are keeping their baby teeth in a box or similar as a memento).
Tattoos are generally frowned upon, as big, colourful tattoos are the hallmark of yakuza. Some public baths and swimming pools won't allow you entrace if you've got a tattoo because of this.
Uttering a thought will breathe life into it, so you should never tell people you wish they would die, because you may actually make it happen by doing so (this belief is called "kotodama").
It's particularly lucky to receive the new year having sex, apparently. ^__- (I've heard this belief described as "hime hajime", but I haven't been able to confirm this is the right term)
It's a bad omen if your shoelaces break while you're tying them, or if something falls down or breaks without an apparent cause.
The first dream of the year will tell you something about what will happen during the rest of the year. It's particularly lucky to dream about eggplants, hawks and/or Mount Fuji.
If you wake up in the middle of the night and can't move, it's because a spirit is sitting on your chest.
It's bad luck to kill a spider in the morning or the evening.
If the first person you meet in the day is a Buddhist priest, you'll have bad luck the entire day.
You should never sleep with your head to the North, or you will have a short life-span (because corpses are placed like that the night before the funeral).
You should never stick your chopsticks vertically into your food because that's the way it's done to make an offering to dead people in funerals. Similarly, you should never pass food from one pair of chopsticks to another because bone fragments of the cremated body are passed from person to person using chopsticks before being placed in an urn.
It is bad luck to step on the cloth border of a tatami mat.
It is bad luck to break a comb or the cloth strap in a geta.
If you line up bottles filled with water around your yard or garden it will keep stray cats away.
If you get hiccups 100 times, you'll die.
If you give your cold to another person, you'll get better.
If you play with fire, you will wet your bed (said to children to scare them off the matches).
Your blood type will determine your personality.