In Japan, there are many custom and behaviours that aren't understood for a foreigner, one of the most notable is the one of "Ladies first", because it doesn't exist at all in there, of course you will find many people that DO have that habit, but in the main part of the situations (or maybe I should say in the most of the cases) this is something that isn't taken with much importance; for example, you can see many guys that open the door without advice, pass before you and leave the door to close (and sometimes, hit your nose)... many to many of you, like me, this is tremendously bad, but is a normal thing in that country.
Next, I will put a small list of some things you must pay attention to before going to Japan.

- There isn't "Ladies first" (sorry for the females out there).

- NEVER arrive late to a meeting/appointment, especially if it is a business meeting. In Japan there's the custom to make everything work like a clock (you can see it on transports, trains, buses, etc.)

- Avoid the excessive visual and physical contact with the person you're talking to. They aren't used to look and being looked at, so this can be interpreted like a provocation, and in the case of physical contact (shaking hands, touching the back) that's practically unknown for a japanese person, so the best thing to do is the reverence they always do.

- Try to not move your hands while talking, this can be taken as an attempt of aggression.

- Never use your fingers to point at a person, if you really need to point at someone, use your whole hand (Fingers joined up and extended, and the palm of your hand up). You just use your fingers to point at things and animals.

- In Japan, often, silence is as useful as a spoken word, many times facial and corporal expressions say more than a word. Smile always.

- Never chew gum at work or in formal ocassions.

- Follow the time, when someone says that work begins at 09.00, THEY DO BEGIN TO WORK at 09.00, not less, not more.

- Avoid to carry many jewels and accesories as well as colourful clothes and much perfume. And even more when it's a business meetings. In Japan there's the belief that people that like to attract attention or wear many things are tied up to the "underworld" (mafia, dealers, etc.).

- In Japan, professional workers or office workers (in the main part) get out of work after their boss or superior has done it. Don't wait to be "free" immediately after it's the time or the finish signal, in many times, they keep on working until their superior has finished.

- The personal presentation cards are indispensable items in a forum reunion, you have to deliver your card extending both hands, presenting the card with the right hand and in position that the person can read the name (never give it with the name looking down, at the other side and with only 1 hand).

- You always have to pick the card with both hands, you have to read it carefully and not keep it immediately in your pocket. Is a custom to have it in the conversation table so you can see it and not have the possibility to mistake the person for another. NEVER keep it in the back pocket of your trousers or pants and even less sit in front of them doing it.

- Is a custom to add "-san" after the name of the person you're talking to, this is aceptable in any case, but with young ladies you have to add "-chan" or "-kun" in the case of guys or subordinates... but you must never use this for your own name.

- Is not nice to complaint about the behaviour of unknown people, the bad service in a restaurant, because there aren't things you liked or because the place where you are is filled with smoking people. In Japan, the topic of non smokers isn't taken seriously... yet.

- Avoid the use of loud voice to attract someone's attention.

- If you have to clean your nose, get out of the room (it's unforgivable to do it in the table). If you are in a place where you can't get out of, turn around and do it in the most careful and silent way, and never use the cloth tissue for this, use the paper tissue.

- Is not common to use the shirt outside your pants.

- In the electric stairs, is a custom to stay in the left side, the right side is reserved for people who have a hurry and have to go up faster.

- Is not a custom to use sarcasm or make sarcastic comments (and they aren't understood either), if you wish to express something, do it clearly, many times when you use sarcasm the listener can think it's a grammar mistake and follow you literally.

- The japanese gesture to say "I" or "Me" is to point at the nose, not the chest.

- The gesture to call someone is with the hand at a side... up or downwards is used for animals.

- The gesture to say "No" is to move the hand in front of the face (as if there was a bad smell).

- The gesture to ask for permission to pass in the middle of 2 people is to put your hand in a vertical way in your chest and move it forward, as if you were cutting something in front of you.

- Japanese buildings generally have thin walls and less isolation, turn down the volume of your TV, and after some hours avoid talking with loud voice, listen to music and flushing the toilet.

- You must use shoes with an easy way to wear and put off when you visit somebody, and never wear socks with holes.

- When you enter a japanese home, you'll get special shoes (スリッパ - Surippa - Slippers). Never enter with sandals to a room with tatami, you must enter without shoes.

- Don't use the slippers outside the house such as in the garden, as well as in the entrance (at the time to put your shoes back on to go).

- Don't enter the bathroom with those slippers, you will find special slippers inside, as well, don't use these outside the bathroom.

- If someone offers you a Yukata (summer kimono), you have to cross the left side over the right side, otherwise it's used only with dead people.

- Is typical to reject any type of help you can get.

- And also, they will reject your help even if it is vital to them lol.

- Traditionally, these offers of help are done 3 times (or more) before they are accepted. In this case is better to express something clearly such as helping someone with the luggage or calling a cab, instead of only wanting "help".

- Japanese women cover their mouth when they laugh (this comes from the buddhist belief that showing a bone is something unpure), if you are a girl you don't have to do this, but you can end like me, doing it without noticing.

- You must take food or drinks (covered and packed as a gift for formal situations) when you visit someone.

- Giving a gift or a present is a very important thing in Japan, if you receive a gift, you have to return the courtesy with a gift of the same value or higher.

- Avoid sending too luxurious or expensive gifts.

- Giving money is normal in weddings and funerals, but they are given in special envelopes for that event (they are sold in any store). This envelopes have a coloured ribbon (natural or printed) for weddings or black for funerals, and you have to write your name and the content always. For this, you must use new bills or in good state, if you use old bills this can be interpreted as a lack of respect.

- After going out on vacation or long trips, it's a custom to take a gift to everybody that work with you (generally a candy or something that can be eaten to be divided into everybody) even though you don't appreciate them. You don't need something expensive anyways.

- Is common to say that what you are giving them is a small thing (even though it isn't).

- In formal circumstances is a lack of courtesy to rip off the envelope or cover of the gifts you receive immediately. You must ask the person who is giving it to you if you can open it in that moment or not.

- When someone visits you the custom is to receive that person with surippa to use inside the house and put the shoes of your guest together so he can put them back easily at the time to leave.

- An expression of courtesy is to follow the person until he/she is out of the house and to see him/her go away.

- There's an old custom where the guest gets offered the seat that is infront of the entrance or looking at the main side of the room. Then, the elder or most important person should seat in front of the guest.

- Another custom (not too old) is that the most important person should seat behind the driver of the car, and in the case of a taxi, the less important person should seat at the side of the driver.

- In Japan, the taxi driver will open and close the door automatically so that the passenger can get on (don't try to open it, just step infront of it). This is only to the back left door of the Taxi.

- Is a custom that japanese people praise the good use of the language (as well as your appearance, looks, etc.) of any foreigner. But you must deny it (even if it's true) so you can show your good will.

- If you stay as a guest in the house of a japanese person, is most probably that you will get offered to use first the bath (ofuro). The custom is to share the warm water with all the family, so in the case of the Ofuro the right thing to do is to use this water just to warm up your body after showering (outside the baththub). When you finish, don't take out the plug of the thub.

Indeed, a lot of these things are very VERY important things. And like I stressed way back in one of my early articles, the one thing that particularly applies to dealing with Japanese people even outside of Japan, is when they compliment you on your Japanese. As Zelda said, you MUST deny it, or you'll be viewed as an arrogant fool!

Also, an interesting perspective on that whole thing is this. Zelda said that you have to deny it even if it's not true (meaning even if your Japanese is good, say it isn't). Well, the person saying that your Japanese is good may not mean it either! In fact, you can assume that when you're no longer complimented on your Japanese, only THEN is it truly good!