Cooler weather in the north
Clothing/Shoes/Weather Gear: Warmer clothing will be needed for cooler temperatures in the north than in the southern islands. Regardless of the summer heat and humidity, or the winter cold, the northern island and Sapporo can be ~10 degrees cooler than neighboring Honshu island. With that, there is also a higher chance of rain in the summer and snow in the winter, so best to be prepared.
Luggage and bags: If you have to walk anywhere it is very difficult with suitcases in the ice!
Clothing/Shoes/Weather Gear: You need waterproof shoes, thermals (including socks) a proper warm jacket that has some waterproof quality and jumpers as well. Shoes need to be abe to be easily walked in on ice! Gloves and a scarf are also a great help!
Packing Requirements Summer
Luggage and bags: It can be hot so try and limit baggage if you need to be walknig a lot with it.
Clothing/Shoes/Weather Gear: It gets very hot and sticky in Summer - make sure you have clothes that will suit the weather - shorts, tshirt etc. At night it can be quite uncomfortably hot also depending on where you are staying.
Toiletries and Medical Supplies: If you don't speak Japanese make sure you have all the medications you might need. Its very hard to find any medicine like panadol brand etc. so if you think you will need it bring it with you.
Miscellaneous: Bring all the items you would normally need for travel
Luggage and bags: Use a small, compact suitcase. Avoid anything large and bulky.
Clothing/Shoes/Weather Gear: My advice is this: pack all the clothes you will need for one week. Then if you need more, in the event you go clubbing, etc. go shopping for some new clothes with your Japanese friends. As a Westerner you'll probably take a size Large. Also get something loose and stretchy, even oversize with short or 3/4 sleeves. The full length sleeves on a typical Japanese jacket or dress may be too short for you.
I will advise that although many young people in Japan are trendy and radical in their styles, most Japanese are conservative dressers. If you go visiting, especially to meet older people, clothing similar to 'dress casual' will go over much better than 'dress down' styles of clothing. If you go up north to Hokkaido, there could be snow as late as March or April. It's quite cold there, sometimes the wind stings. Bring a muffler to wrap around your face and a good sturdy pair of boots like Timberlands.
Toiletries and Medical Supplies: Bring lip balm and lotion for dry skin. The winds and cold in Sapporo can be harsh.
Miscellaneous: Cash, credit cards & a Japanese phrase book. SUSUKINO is Sapporo's version of Ginza, a shopping center underground with covered arcades that slide back their roofs in the summer! Japanese ingenuity at its apex!