Methods

Skydome
Light is coming from a hemispherical dome above the horizon.


Environment
Light is coming from everywhere.


Adaptive Sampling

When "Override current settings" is activated, adaptive sampling is used, which is better for ambient occlusion than the other methods.
Settings from -1/2 to 0/3 are usually sufficient.


Subdivs

8 Subdivs
Noise is clearly visible.


16 Subdivs
Noise is much reduced, rendertime doubles.


For tips check out the faq section.