The electricity is 100V which is slower than the American 120V, but the plugs are very similar so you can use electrical appliances without a plug adaptor, but they may operate more slowly or not get as hot.
Pack lightly. I can't stress this enough if you plan to use public transportation at all. We used the smaller carry-on size luggage with wheels, but we saw plenty of tourists with huge suitcases trying to get them up and down staircases or into train compartments and they just don't fit. One family was forced to keep their baggage in the entryway of the train with a companion to watch it for over an hour because it wouldn't fit into the train seats or in the overhead bins. You can send out laundry at hotels or wash it in the sink and hang to dry. All the hotels we used had a washing line to dry items over the bath. All of our hotels provided toiletries including toothbrush, so it was one less thing to pack. They also provided robes and pajamas, which will only fit if you aren't much taller than 5'7" or heavier than 175 lbs. You will want to pack some nice socks though as you will remove shoes often if you are visiting temples. I recommend merino wool clothing because you can wear it more than one day and it doesn't retain odors. The travex line by Eddie Bauer or Columbia's travel clothing has been some of our favorites. It's also great to keep you warm/cool depending on weather. Layering is also helpful. Mornings can be cool, but afternoons warm up quite a bit in springtime. The Japanese wear nice clothing- clean with no holes. You won't really want to be seen in public in torn jeans, shorts, sweat pants, flip flops, etc. Save the casual wear for the beach.
If you will be bringing small children, a stroller is helpful, but it can't be the supersize model. You need an umbrella stroller that folds up small because you will have to fold it and eject the child every time you get on a train or bus. We use the Maclaren Volo when we travel. It's extremely lightweight and holds a child up to 55 lbs, although our son is now 75 lbs and he still fits, but his knees are up on his chest. In spite of it being too small, he prefers using it when we have to walk long distances. Also a good idea to bring a small daypack to carry your stuff with you during the day. Many public toilets don't have towels, so a pack of wet wipes is useful. We usually carry Clif bars with us on trips too so even if you can't find a restaurant, you won't get too hungry. They go through airport security better than trail mix. Having pockets in whatever you wear is helpful too so you have a quick place to store change and subway tickets or your JR pass. You have to show the pass or ticket when you enter and exit the train.
Bring cash. There were very few places where you could use credit cards not issued in Japan. You could naturally use it to pay at hotels and hotel restaurants, but all the shopping we did required Japanese yen. You can bring a debit card and get yen from ATMs at 7-11 stores. There were also banks and hotels that exchanged dollars, but you will pay a fee. And be sure to call your credit card/bank card number before you travel to preauthorize use in a foreign country. You want to ensure you can use it when you need to have access to your money. Be aware that most cards charge at least 1-5% per transaction, so you aren't surprised by excessive fees when you get home.
Wifi is hard to find and not cheap. Most 7-11s have it and it's free, but your websites may show up with Japanese characters once you log in. If you plan to use your iPad or smart phone, you can pre-purchase time from your carrier before you enter the country and it may work out cheaper and easier. If you opt for the hotel wifi, you generally have to pay about $20 per day.
If you plan to travel to more than one city, it's beneficial to purchase a Japan Rail pass, but you have to purchase it before you leave home. Then it comes via mail, and you activate it the first day you travel by train in Japan. You can read more about the pass and get one at the Japan Rail website. We were there for 8 days, but had a 7 day pass. We used this pass for local transportation in Tokyo and Kyoto as well. The first day we used a shuttle bus to get from the airport, which was very easy. Once you are out of baggage claim and customs, there is an area before you step outside where you can purchase ground transportation. They speak English and can tell you what options are available. It is quite a long way from the airport to the center of Tokyo. Our ride via bus was nearly 2 hours. Once you've had a few days to practice using the public transportation, it will be much easier. There are maps available in English and Japanese at the stations, but you can download maps to your smartphone too for preplanning. When you're ready to go back to the airport, ask about reserving a seat on the Narita express- the express train from Tokyo to Narita airport. They even serve snacks.
You can use any number of travel guides to learn more about Japan and to plan your trip. I usually prefer using Frommers guide to read about places in advance, and they have downloadable books on itunes under apps, or you can use Amazon to put it on your kindle. When I'm walking around doing the sightseeing, I prefer the DK Eyewitness series for their amazing maps, photos, and general information that you need in a jiffy. We did use a few of Frommer's city walking tours which are available free on their website (you can just print out a few pages) to plan our days, but used the DK guide as a supplement, which worked well. The most important thing is to have a plan. If you just show up at a local train station with no plan, you will be overwhelmed. Usually the concierge or local information desk at the train station has some ideas to hone your plan, but you should know which attractions are high on your list. You can also search Facebook for Japan travel pages. I subscribed to several and then we got updates about where the cherry blossoms were blooming in real time as we traveled. Because of these updates, we attended a night time festival that turned out to be well worth it.
Traveling to Japan via airlines is a personal choice, but we found that using ANA was a dream compared to any US airlines, which is usually the case when you travel long distances. Most foreign airlines are subsidized by their governments, which means they offer more amenities like leg room, nicer food, snacks, and have young kind flight attendants. The aircraft are often newer too. Of course, price is a big consideration, but if prices are similar, I'd go with the Japanese airline. If you can, choose a window seat or at least get up during the flight and look out from the windows near the rear. You'll be flying over Alaska and may have the opportunity to see Denali national park or Fairbanks. I thought the views were amazing. If you have a TV at your seat, there's usually a map option so you can follow the path of the aircraft. It will let you know when you're over Alaska.
Finding a hotel can be easy using any travel website like expedia, kayak, trip advisor, etc. We ended up using "my tokyo guide", a company I found on Facebook, to find ours because we were traveling last minute and had trouble finding accommodations at all. Often the upper tier hotels hold back rooms for travel agents so you end up paying more, but you will get a nice room. In both cases we stayed right near the local train station for our Kyoto and Tokyo stays. It was so convenient. We asked for hotels with pools because Omar loves to swim, however, in Japan the pools are for adults only. None of us ended up using them due to the stringent rules. Jake did use the health clubs at the hotels though. I was happy that my tokyo guide suggested an overnight at an authentic Japanese Inn. It turned out to be the most interesting, relaxing night we spent. You can read more about that at my Japanese Inn blog post. I would recommend this to anyone going to Japan so you get to see what the Japanese enjoy. The sushi was fresh and amazing too. When we do foreign travel, we always try to add breakfast with our hotel if it's available. Sometimes it costs a bit more, but it is so much easier to rise and shine when you know coffee is just down at the lobby. When you're suffering jet lag in a foreign city and the weather is crummy, you don't really want to search for your breakfast. Once you eat, you can set out much happier.
If you'll be visiting Japanese people while you travel, you must bring gifts. We brought American chocolates, but you can also pick up food or flower gifts while you're there in Japan. Small hand towels are also welcome because as I mentioned earlier, there often aren't towels in public bathrooms so people carry towels with them.
I hope these simple tips help you when visiting Japan. Feel free to ask questions if you need any additional help!