With exercise. A combination of physical exercise plus mental challenges does appear to slow the onset and progress of dementia (but cannot prevent it). See http://www.Ncbi.Nlm.Nih.Gov/pmc/articles/pmc1151037/ the physical activity component is crucial, as the brain is itself a physical organ. Indeed it consumes 25% of our energy production, so it the organ most sensitive to energy depletion.
Controversial. At one time we though this was true-paricularly in one study that i recall where aged educated nuns were being studied but i think that theory has sort of fizzled.
Some protection. Religious orders study out of chicago, did suggest some protection in better educated and more intelligent people due to cognitive reserve, but when such individuals get dementia, the downhill course is far more rapid.