Hi there,
I'm not very experienced, but I just got back from my second trip to Japan last week and did learn a few things. I may be pretty far off, but this is what I observed.
First off, it's a good idea to try and read all the info on this forum. There's lots of good stuff there. It did help me a bit. Knowing japanese helps a lot. There will be places that say "Japanese only". I got turned away from quite a few places. The young hot girls will most likely be the ones for Japanese only. Prices seem to be a bit steep too. If you don't know japanese, you should try to find a friend who can show you around.
There are many places, but obviously they will all be in Japanese, so it will be harder to find if you can't read the signs. You will probably find quite a few of them close to the train stations. Just look for the more shady looking areas. Usually there's a lot of bars in the area too.
Look for places that have "lunch time" specials. I got a great deal for 5,000 yen 30 min. Non FS, but still fun (almost all the places in Japan are not FS, since FS is illegal).
Watch out for the chinese massage girls. I guess it's because it was winter, but they were all wearing these puffy jackets. They generally are standing at corners and will walk up to people saying "masaji". Though those may be the easiest to find and most foreigner friendly, they are also very very expensive. They will try to lure you with a 3,000 or 6000 yen massage. Once you reach the establishment then they will try to squeeze as much money out of you as they can. After having given them 6,000 yen they will ask you if you want a "special massage" for 10,000 yen. They will keep this up. 10,000 more to touch, 10,000 more for cbj, 30,000 more for fs. but by then that's already 66,000. (I didn't go for it, waaaaay to expensive) If you say you don't have the money they will ask if you have a credit card, or where you're staying. I tried a second time somewhere else but couldn't get a good deal so I stopped at the regular massage. Maybe if you're smart and firm in the beginning you can negotiate something better from the start. I just said, forget those people already.
Escorts are expensive. Around 35,000 - 40,000 yen, but you'll probably get what you want.