Um - drive faster?
Seriously though - rallies are pretty difficult in the GT series. I've always battled with Tahiti especially.
I find that you need to work on getting a 'flow' going where you're comfortable with both accelerating and braking with the car unstable, i.e. sideways, and you also need to learn how to hold the car at a particular angle on the throttle.
Watch the demos from the first person perspective a couple of times and see how the AI uses a combination of brakes, acceleration and steering to hold and position the car, and get a feel for what you need to be doing where.
Good luck!