My latest, attached FAQ now includes a very long answer to "How do I increase my stats?"
At the start of the game, you are asked which stat is your best: looks, charm, hacking, strength or endurance. This will give you a head start on the stat you choose. From then on, you must work on gaining points toward building up each stat. Your phone can tell you how much you have done toward geting an increase in each stat. By using the stat, you may get points toward increasing the stat. When figuring out how many points are needed, remember that for all stats there have diminishing returns, so that it takes more effort to increase a stat at each level. Don't slack off until a stat is maxed out. Unfortunately, your phone doesn't record how many training points you have for strength and endurance, just the number of times you have worked out.
To increase your charm and hacking skills, choose the reading option while in your room. Also, being charming with the girls will show up on your phone as training points and go toward increasing your charm. Sneaking into a girl's room to peep on her in the shower will also show up on your
phone as training points under hacking.
To increase your looks, you must apply beauty cream. Using the hair style gel, will get you a temporary increase, for impressing Yumiko at last call at the nightbar so that she takes off her top and has a drink with you.
To increase your endurance, you must swim in the pool or run on the beach or on the treadmill at the gym. Remember, doing more than four or less than three endurance training sessions per week will slow down your progress. Check your phone to see how many sessions you've done for the
week to make sure you train between three and four sessions each week.
To increase your strength, you can help the girls with their weight lifting or train in the gym by yourself or with Jennifer. Jennifer explains that it is better to work out with her and will get you faster gains, but you must also concentrate on your training with her. If you get distracted by her breasts, then it is just like a normal training session. Doing two or less, or five or more sessions will slow your progress, so check your phone to make sure you are doing three to four sessions per week.
The way training points work for your strength attribute is that there is a 50% chance to gain one strength point for lifting weights with a girl. With 1.4.2, there is a 50% chance to get two training points when you train with Jennifer and fully concentrate on the training. If your current strength level is 2 or below, you'll need 4 or more strength training points to gain 1 level of strength. Between 3 and 5, you'll need 6 or more strength training points. Between 6 and 8, you'll need 9 training points, and for 9 and above, you'll need 12 training points. If you train 5 or more times or 2 or less times, you'll only gain 1 strength training point for the week. If you don't train at all, you lose 1 strength train point. Which could eventually lose you 1 level of strength. Thus, it is easy to gain levels up until you hit a 6 strength, whereupon it becomes harder after that.
The other stats increase similarly, and I hope that demystifies the process for you.