I seem to be totally unable to use RWD cars, and the top tier cars are almost all RWD.
I can slide into a turn at 90 MPH and correct myself in a spinout no problem if it's FWD, but sharp turns or a gust of wind carrying a hint of ill intent will make my Bullet GT do its best impression of a newspaper caught in an updraft as it uproots off the track and homes in on the nearest tree.

I know it's not the car's fault, but I just cannot seem to crack the code. Any tips?
As you might've guessed, I'm not a racing game fanatic, and Flatout's the one racer I know and love.

Use the e-brake before you get to the corner. Only tap it as you just want your ass end to come out and hit the gas again to get you going. It takes practice but you'll get it.

First of all I suggest to use an analog controller and automatic gears.
Problems with driving RWD cars mostly occur due to being too hard on (and off) the throttle.
As you probably already know in racing it is very important to follow the (imaginary) raceline, now imagine you're approaching a left hand 90º corner:

- when you approach the lefthander position your car to the right of the track
- before braking ease up on the throttle, if you suddenly go from full throttle to full braking this can seriously unbalance the car (which means skidding) depending on things like suspension stiffness, roadcondition etc.
- gradually brake as hard as you can, the more straight you drive the harder you can brake.
- now you will reach the point where you will need to turn, this is important don't turn in too early or too late as you won't be able to make the bend properly, now gradually ease on the brakes and and steer the wheel and if your doing it okay (which means you haven't spun by now which means you were still braking too hard) you have released the brakes just before you reach the inside of the corner which is called the apex.
- once you're at the apex it is time to (carefully) apply some throttle again, with less powerfull cars and a lot of grip you can apply more, powerfull cars and not so much grip you often can only give full throttle when your car is driving straight, again you will notice if you applied too much throttle when you spun (oversteered) the car, steer the car while keeping a nice radius to the rightside of the track and you should be at full throttle when you're driving straight again.

Now in FlatOut there is a lot of mess that can litter the track, personally I try to avoid most of it as this keeps me out of trouble and when you're driving through debris with full throttle one of the rear wheels will skid while the other has full grip and this will unbalance the car, so again the advice is ease up on the throttle so grip and skid will even themselves out somewhat (at least to a manageble level).