I read about one tip once, make sure all your drivers have heated temper.
This makes them drive good even when they are chased and the heat meter goes up (i don't know for sure, but worked better in my second account, first account i had all sorts of drivers, calm and heatet drivers)
Too bad there is no Season event anymore.
i did the first with the B-spec online season events.
After that, used the Redbull you get at level 35
Drive the B-spec career events until you have acces to the 24hrs races.
select 4 of your high tempered drivers, start race, shut down your controller and tv.
Next day, switch on tv and controller after 24hrs, and start again, or wait untill you finish gaming.
You also could choose a 8hrs race, if you work for a living, but that takes longer.
A zero level b-spec driver gets up to level 18 with a 24hrs race.
It will take days, maybe weeks, but you get there.
The biggest problem is, to get your driver's full career trophy.
To be honest, i will never start a new GT5 career, without online season events, it will take weeks and months, to boost up the levels.
Ow yeah, forgot to mention, i notice that the career level of your driver decreases when you don't use him for weeks or days, keeps you from getting that career trophy.
And i noticed, that level 40 b-spec drivers, cool down much faster during a race, giving the other cars the chance to catch up again (not when you use the redbull car!!).