In order to get to the good/best ending of a girl, it is a good idea to think about what the particular girl would like you to do and behave according to this (in other words: you have to roleplay Takayuki and keep in mind, that he has fallen completely for the girl you want to have a good/best ending with).
Vital hint: On the first day, there will be two questions that Takayuki has to answer in any scenario. The answers to these questions determine a lot about which endings will be possible, because they determine the general attitude of Takayuki towards the other characters (O.K., only the female ones). When answering the questions, it is important to keep in mind that the walls in the ryokan are thin (looks like wood and paper) and that the other characters might talk to each other behind your back. Further, it is important to answer these question honestly (from a roleplaying point of view), because the game has no sense of humour, nor of sarcasm regarding these questions/answers.
This pair plays no part on the first day, because they arrive at the second day. They act as an emergency-brake in the storyline: If you have done everything right to get to a girl, they would give you the opportunity to stray from your chosen path and get their ending instead (O.K., this is not as simple as I put it here, but I restricted this guide to the first day and will not tell about the following days).
(Webmistress addition: small hints on Keiko-san and Misato-sama: Be interested in their lifestyle, let them have whatever they want, but don't let them go too far.)
It would be a good idea to be at the reception when Yuki arrives at the inn. Later in the evening, there wil be an opportunity to peek at her while she changes clothes.
In the morning, she appears while Takayuki is still busy with a task. At this time, they have a talk about what she is planning to draw. Also, Takayuki has to meet her this day two times in the bathing area.
Takayuki will meet her somewhere, before she arrives at the inn. After her arrival, Takayuki has to remember this first encounter. Later in the evening, Kyoka will call for alcoholic drinks.
In the afternoon, Takayuki will lend her his jacket for some reason. After supper, Takayuki will ask Ume-san to take care of a task, because he himself is busy. Later on, Takayuki will take credit for Ume-san's work.
While returning to the inn, Takayuki and Kumi will get very close physically. After supper, Takayuki will ask Ume-san to take care of a task, because he himself is busy. Later on, Takayuki will take credit for Ume-sans work. (Yes these are the same events as in the Natsuki-storyline)
In the afternoon, Takayuki will lend her his jacket for some reason. After supper, Takayuki will get a good eyeful of the trio and a big bruise.
Attention! The wrong answer to Kumis question will completely block the path to Makoto-sans ending. After bringing the trio to their room, Takayuki will help Makoto-san, as she asked him to. After supper, Makoto-san will praise Takayuki for completing a task correctly. Later in the evening, Takayuki has a close call with death while watching Makoto-san (don't dare to make a noise, hold the breath and get nearly a heartstroke, to say the least).
Two times this day, Takayuki will work together with Ayumi-chan. Very late in the evening, he will become her knight in shining armor (even though, Makoto-san helps him in this matter).
Ume-san bad ending:
This is not a girls good/best ending, but the bad ending with Ume-san. Most players (including myself) had troubles in finding this ending.
This ending is not a nice one! (Maybe I am silly, pointing this out. It is called a bad ending, after all.) Some players who desperately searched for if felt betrayed (to say the least) when they finally got to it, so it might be a good idea to stay away from this ending, because it might spoil the overall good impression, the game leaves in it's other endings, even the normal ending.
How to get there anyway:
Instead of the guideline "do, what would please the girl", for this ending, the guideline "act like a lazy bastard" has to be applied. Try to stick to your usual work (cleaning the bath), or better, take a nap instead of working. Whenever asked to do something, refuse, if possible. Don't ever give the impression, you might be interested in one girl particularly, but in all girls.
Critical event on the first day:
Makoto-san will make a request that has to be refused.
(Webmistress addition: There is one time you must insist on doing some work on the second day.)