Here's a tip
Actually, here's not a tip. Japan is not a tipping society so you don't need to tip in restaurants, taxis, etc. In fact, my daughter actually had the waitress chasing them down the street once when she left a tip on the table. The waitress thought she had forgotten to take her money.
Some higher class restaurants may add a 10% service charge, but these are few and far between.
Most hotel porters won't accept tips - they're not allowed to in major hotels - but some may accept tips for helping you take your bags to the room.
The major acception to the no tipping rule is in traditional Japanese ryokan (inns). The custom here is to place a few thousand yen (amount depends on the number of nights you are staying and the quality level of the inn) - say, 2,000 - 3,000 or so, into an envelope and give it to the person who will be in charge of taking care of your room and your comfort during your stay. This person will usually be the one who also serves your dinner in your room (and if breakfast is in the room, your breakfast too). Give this tip at the beginning of your stay after the staff has explained the procedures for dinner, breakfast, the bath, etc. to you.