After a bit more play/testing, here are my suggestions/tips for making money, aimed mostly at new players:
1- never have more than 3 girls serving customers at the same time (unless there's a point where a LOT more customers shows up). Since not all of them will be able to recoup their salaries, you'll end up loosing money (either by making less profit, or taking an actual loss in case of bad nights.)
2-Upgrade the Stage, and entertain room as early as you can, those generates revenues, which can make a whole lot of difference very quickly.
3- Save before going to the night service. If you've got attacks/bad events, or simply a terrible night, reload, and you won't get them again, unless you're particularly unlucky.
4- Don't bother with the dojo (see tip above, this negates the need for it, while waiting for the dev to check if something's wrong with the attack rate, and their costs.)
5- Don't bother with the onsen until you start making proper money. Until then, simply avoid sick customers, the many days loss you'll take, is simply not worth it.
6- train the girls in a single category to 100% at first, then train them in rough sex. ONLY get them on rough sex with customer matching their 100% skill, or they'll get used up even with rough at 100%
7- Do not bother training any skill above B until very very very late in the game. There are simply no customers looking for A ranking for a very long time (I suspect this is linked to your reputation, so as long as it takes this long to increase, you'll just end up wasting time training the girls.)
8- Once you've got several girls trained to 100% rough, you can start increasing another skill to 100%, although if your customers are still not above B requests, it does not seem to be worth it.
9- You'll only get about 6 customers in a night. Even with a full upgrade on passives/recoveries, a girl can serve maybe 2 customers, 3 if you're extremely lucky right at the start of the night. Don't bother TOO much saving them for "later" in the night, you'll more than likely end up loosing money that way.
10- You can buy improvements, although, I'm not entirely sure they're worth it at the moment. Their costs, and the time at which you can get them, render some of them not that worth getting (the skills one especially.) By the time you can afford them, most of your girls will be near the top of everything already. The extra cost saving/revenue making and so on, are worth getting in the long term, but not worth getting early on, as 10% of a very small revenue, is negligible.
11- The skills body traits and personality, seem to be a waste of talent points. This is due to the sheer amount of customer options available. Getting the right girl, with the right customer for those to apply, seems to be so rare, in my opinion, they're not worth getting, especially at this stage in the game.
Note: most of the tips above, are only valid on the current version ( 0-7-4). I strongly suspect the dev will be making changes and adjustments on several things listed above.