In leaving the states you are allowed to take $10,000 per person out of the country without declaring it. When you return from Japan and you have bought something new, as Frosty mentioned above, take the tags off and say that you brought it with you. This especially holds true for electronic gear like cameras and such. Ditch the boxes and wrappings and at least use it/wear it while you are there.
Coming back to the states you are allowed to take back $400 in merchandise bought overseas without the payment of duty. Anything over that and you will be subject to pay the tax. However, many items can be bought at the "Duty Free" shops at the airports in Japan and the US, but they are really not that big of a bargain.
I do believe there are some "Duty Free" Japanese electronics stores located in Akihabara, but I am not 100% positive. Someone will chime in on that though.
One final note, DO NOT buy your yen at the airport in the US or at a US bank and don't buy yen travellers cheques as they are a rip-off. You will get a far better rate buying your yen at Narita airport when you exit customs. You will get at least 7 more yen/$ and it really adds up when you're exchanging a few thousand dollars. If possible bring cash, preferably in $100 bills as you will get the best rate over travellers cheques. Japan is a very safe country and you will be safe carrying large amounts of money with you. But if you are still unsure you can take some travellers cheques with you.
Have fun and I hope you enjoy yourself.