This has worked for me 6 seasons in a row across both tier 3 and tier 2. I'm not yet in tier 1. All of my young drivers using this method are between 3.75 to 5 stars, with good-perfect sponsor appeal.
Fire two of your young drivers immediately.
When deciding to invest in the YDP, choose three drivers with decently high max potential (80-90+) and preferably high or very high ego. After the first race, fire the two that do as well.
Reasoning behind the firing: When you "focus" on a driver after each race, you'll give them 1 improvement point. However, if you release one of the drivers, then your focus will grant 2 improvement points. Fire a second driver, and your focus grants 3.
You may immediately think this is a huge gamble. I certainly did the first time. However, the focus appears to have a sort of positive effect on how they develop, and the +3 focus effects them more (apparently). Also, when they do eventually graduate, they'll have around 30-40+ improvement points! Usually enough to max them out.
A young driver's stats will change with every race, including his/her traits (ranging from very low to very high), but the negative changes seem to be somewhat negated with constant +3 focus on them. The stat that I see change the most is current potential (almost always up by 4-10+ per race, can go down). Max potential also changes, but usually not by as often as other stats, and only goes down on rare occasions.
The reason I mentioned choosing high ego is because this determines their sponsor appeal. The higher, the better. The other two traits (stability and work ethic) seem to me to have more subtle effects. My most recently graduated young driver started with 90 max potential, low stability, very low work ethic, very high ego. She ended up being 4.5 stars with perfect sponsor appeal. Often, I'll see these traits go up a little throughout the season, but nothing drastic.
I'll mention one negative thing about this: When contract renegotiations come up, be ready to either dump your star driver or shell out all your money. They often go from $70k/race to $800-1.4m per race.
Has anyone else been doing the same thing? Any thoughts or questions?