The biggest problem most people have is that they tilt the seat base so the knees are higher than the hips - this forces the body into an unnatural position.
Start with the seat base as close to level as you can get it, and adjust everything else around it:
- Slide seat forwards/backwards so that when the clutch pedal is fully depressed, your knee is not quite locked.
- Seat height is purely personal preference - personally I like to be able to see over the top of the steering wheel, some people obviously don't
- Seat back tilted so that with your shoulders still in contact with the seat you can hold the top of the steering wheel, with your elbows slightly bent.
- Squab adjusted to support as much of your thighs as possible, without hitting the back of your knees.