About skills I agree with zanzibar, but about Strenght, Agility...etc. There are 2 ways:
1) PP-s can be used only for stats, but this isn\'t very realistic, becouse of the same problem as with skills, you fight and than raise Int. for example...
2) Stats can be trained by activities, running around with heavy backpack increases endurance and strenght.
Fighting increases Strenght and Agility.
Using magic increases mental stats: every way increases the stat that it\'s dependant on...
But when stats rise by doing something the increase should be small enough...
And faster training could be done with trainers: in Gym\'s, nightschools

, with jogging, etc...
And also there could be skill decreases, for exaple, if you only fight night and day, your mental skills would start to decrease(but at very slow rate) and the other way around (this could cut down the PL-ing, but there are always those who don\'t care...),
so you would have to do different things to keep your char balanced.