Desire night scene. You must first see all of her nighties, then she will show you the blue sexy nightie when she tells you she will repay the favor. What she wears is somewhat random, but it can change. Visit her, if it isn't the right lingerie, then PgUp until you are back where you came from. Then change your location and visit again, repeat until you get the desired result.
If you train in either strength or endurance 5 times a week, you will only get 1 point towards increasing your stat; so, only train a max of 4 times per week in either stat. Your phone keeps track of the number of times you have trained, but not your points (as it does for charm, hack and looks). Talk to Jennifer about the optimum training regime. Always strength train with Jennifer and concentrate on your training, not on her face. This will usually yield 2 training points. If you need more points, help the girls lift weights, this will get you 1 training point each time. As your stat increases, the more training points you will need in order to increase it. You can jog on the beach or in the gym or swim laps to get training points for endurance. You can usually only train once per day, but if you get a massage, you can get in another session of either strength or endurance. Also, if you meet with Alice on the sunbed and look at her face, she may kiss you and get you so horny that you swim laps to cool off.