Ackerman settings can make a good deal of improvements, and I personally like to use this area on the kart for fine tuning.  You might also find that it helps to have extra sets of tie-rods that are already setup for different Ackerman positions, make sure you mark which side of the kart they go on as not all karts are perfectly symmetrical when it comes to the placement of the steering shaft.

What is Ackerman:  This is a fun one!  The concept is to have all four wheels rolling around a common point during a turn.  This means that the inside front wheel needs to be turned more compared to the outside front wheel.  Changing the amount of Ackerman then changes how much the inside front wheel turns compared to the outside.

By increasing Ackerman the toe angle increases (toe-out) as you turn the steering wheel, while neutral Ackerman results in the wheels staying nearly the same.  Increased Ackerman results in greater turn-in for your kart, also creating more forces on the inside front tire and changes the weight jack effect on the chassis (see this article on weight jacking - coming soon).

Ackerman can be changed on the kart in two areas, both on the spindle arms (some karts only have one hole), and the steering shaft as seen in the image above.

To increase the amount of Ackerman you would move the tie-rods to the inner hole on the spindle arm, while moving it to the outer hole reduces this effect.  Some steering shafts will have a range of holes to use, using the outer holes will increase the Ackerman, while the inner holes reduce it.  The steering shaft pictured also allows you to reduce the amount of steering movement it takes by moving the tie-rods up or down as well.

So how do we put this to use?

Setup Tip: The rear of my kart slides/oversteers upon entry to the corner

Reduce the Ackerman on the spindles by one hole.

Setup Tip: My kart has too much front steering or bite on turn-in

Reduce the Ackerman on the spindles by one hole.

Opposite is used for understeer or lack of bite. 

If you have the ability to use upper/middle/lower holes on the steering shaft like pictured.  Using the upper hole as shown will result in less steering input compared to your hand movements, while using the lower holes will result in a faster steering rate.

Author's Notes:

It is important to use controlled hand movements, don't be jerky, concentrate on smooth inputs in your steering to avoid steering too much initially.  This will improve your lap-times more than you realize, when you are correcting for too much steering input you are most likely sliding and therefore scrubbing off speed.