Okay, only partway through "perfecting" the game, but here's what I've figured out so far.
-Save/load options are a hidden bar in the upper left, no suprise. You can save anywhere, anytime- if you don't know the language or whatever, just save before you make a decision in case you pick the wrong one.
-If you want to skip through the game a bit easier without so much risk of accidentally picking something, you can use the spacebar to speed through text- it's easier to stop in time that way, though you may still hit an option occasionally.
-Time in the game is limited, so you can lose if you try. Generally, when sitting in your room, pick the first of the two options in the initial choices; the other basically just wastes time.
-Whichever girl's storyline you are currently on is the one displayed on the loading screens that occasionally happen- this is an easy way to tell where you're going. Also, the choices are usually listed in the same order, as follows:
--Red-haired girl (warning: quite a bit of [censored] in her plotline)
--Purple-hair girl (severe **41**, obviously; haven't explored this yet)
--Yellow-hair girl (dickgirl stuff here)
--Green-hair girl (extremely busty and the plot only makes her more so and adds milk; this is probably the one you all want)
-In the select screen in your office, after the above choices there are View Profile and Previous Screen choices. In some cases, options are greyed out when you can't chose them or even missing entirely.
-You always have to go through the first "transformation" bit with the red-haired girl in the beginning; after that, you are more or less free to pick whom you choose. Judging by the CG view screen at the main menu (third option, allows you to pick between the stills and scene replays) all of the characters noted above can be chosen. Also, the older purple-haired doctor that you talk to on a regular basis seems to be available, but I'm not sure how to "unlock" her. Possibilities include beating the game with all the characters, getting a "perfect" game somehow (i.e. picking all the right options, etc), ignoring all the chances to do anything with the other girls, or something obscure with your name or the like.
-One last thing: your "progress" on a girl is denoted by the bar you see occasionally when you make choices. It starts at 40% and goes up 10% each time you make a "right" choice, generally by being a complete asshole. You need a girl at 90% to beat the game, it seems.
The game's explanations of things are mildly complicated, so I won't go into things here because I can't read a lot of them without the aid of a dictionary. I may try to write up some kind of synopsis of things at some point, if I ever have the time. Don't hold your breath, though.
-SMAP
"Did any ladies call and say anything about being late? Or about a bottomless cliff?"
"Yeah, about a million ladies called and said they'd be a few minutes late."
"Oh, good."
"Jeez, I was just kidding, no ladies called. But Homsar called and said he was a million ladies tall."