Suspension fault finding

Front wheel chatters entering a corner, the problem disappearing as soon as you let the brakes off or when power is applied.
The fork is working too low in its travel.
Apply more preload
Change to a harder spring
Reduce oil level if a tie wrap on the fork tube indicates a lot of stroke is unused
Check fork friction (stiction)
Rear ride height is too high - possibly too much spring preload. Reduce ride height.

Front wheel skips during heavy braking
Forks are bottoming out
Increase spring stiffness
Lower oil level if a lot of stroke remains

Front end feels vague mid corner
Poor damping control
Insufficient rebound damping - increase
If suspension feels harsh it may be too much rebound or compression damping - reduce

Front end loses grip exiting corners
Insufficent spring compliance or damping control
Increase front rebound damping
Reduce front spring preload
Increase rear compression damping or spring rate

Unfortunately the rear shock set-up affects the behaviour of the front end in some circumstances, so there is often no easy answer to a particular handling problem. The most important thing to remember is never change more than one thing at a time .... Always test the bike after any change in the setup to see what affect it has. And make sure you note down your original settings, so if it all goes haywire you can start again from scratch.

Preload controls ride height. It does not make the suspension harder or softer. What it does do though is alter the angle of the steering, which affects turning speed, and it also changes the weight distribution, which can affect front/rear grip.

Front Preload:
Increase it to put more weight on rear to get better rear traction, or stop the bike diving on the brakes. Too much will cause slow turning and possible front lock ups on braking.
Decrease it to get more feel from front tyre in corner. Puts more weight on front which increases front traction. Too little will cause dive bombing.

Rear Preload:
Increase it to quicken the steering. Too much will cause loss of traction as bike skips over dips in road, and can make the bike generally unstable.
Decrease it to improve stability. Too little and the bike will be slower turning and it will squat when you accelerate, leading to tank slappers.

Compression damping:
This affects the speed at which the shock can compress, under accelerating (rear) or braking (front), and also when riding over bumps. High speed damping usually operated by blowing the shim stack, allowing quick release of oil to compensate for a sudden shock like a pot-hole. Low speed damping controlled by the shims, allowing very subtle oil flow according to suspension loads.

Front Compression:
Increasing will remove some of the dive under heavy braking. Also it will increase the feedback and soak up small bumps better. Too much will cause instability, a rough ride, and it will reduce the grip available at the front.
Decreasing will smooth the ride. Too little will cause excessive front tyre loading and can have the bike standing up on the brakes mid corner.

Rear Compression:
Increase to make the bike run true and hold a line around a corner. Too much and the bike could launch you into space after a bump, and the rear tyre will lose traction on undulations.
Decrease compression to smoothen the ride. Too little will allow the bike to squat under power and run wide in corners.

Rebound damping:
This affects the speed at which a shock rebounds, which allows the wheel to return to the ground after hitting a bump.

Front Rebound:
Increase the damping to help the bike hold a line, and help front stability. Too much will gradually jack the front down as each bump compresses the front further and further. Also it will cause loss of feel and a tendency for the front to wash-out.
Decrease to help the front react to bumps, and to keep it in contact with the tarmac. Too little will decrease front stability.

Rear Rebound:
Increasing will help to stop the rear bouncing around, and the front "pushing" in corners. To much will cause front end shake and overwork the rear tyre.
Decreasing will smoothen the ride on a bumpy road, and decrease the rears tendency to step out on a bumpy corner. Too little rebound will cause running wide on corners.