We all know you can Build Muscle Mass through strength training, lifting weights and other forms of exercise. What is much less well known is the fact that you can increase the mass of your brain through "brain exercises" as well. In fact, it is not too far off to think of your brain as a muscle - yes, your wife and/or boss may quickly agree - that, if properly exercised can quite literally grow. I am saying the brain "grow" or "get bigger" but, in fact, the brain actually becomes denser with more grey matter in the same area. This is the equivalent of putting on mass as a bodybuilder because there are truly more neurons and synapses in the same unit space.
Neurophysiologists call this remarkable aspect of the brain to build (or rewire or rebuild in some cases) "plasticity". Examples abound, but here are a few interesting studies:
1. London Taxi Drivers. London taxi drivers have a very demanding job in terms of visual spacial memory and processing requirements. After all, London is a large and complex city to navigate and remember. Studies have shown that taxi drivers have a signficantly larger-than-normal posterior hippocampus.  The hippocampus is best known for its involvement in memory storage, but the posterior section is used for spatial navigation. What is even stronger evidence is the fact that this relationship was dose dependent, i.e. the longer the time the person was a taxi driver, the bigger his posterior hippocampus.
2. Mathematicians. Working mathmaticians, all professors, had their heads examined - sorry, I couldn't resist - and were found to have several key areas of their parietal lobe enlargened over controls. As with taxi drivers, it was dose dependent, i.e. the longer the time working as a mathematician, the bigger this part of their brain. 
3. Musicians. Musical training, as we have already documented in How to Raise the IQ of Your Children, is an incredible brain builder. One study found that professional musicians had, on average, a 130% bigger Heschl's gyrus, which is involved in hearing and auditory processing, than non-musician controls.  Not 30% mind you - 130%! Sure, well-developed biceps are nice, but wouldn't it be nice to have almost one-and-a-half times the brain density you had before? The Broca area of musicians has also been found to be significantly larger as well. 
Is bigger better? Well, we would certainly argue that that is the case when it comes to your brain. By the way, this likely gives you more buffer during your senior years. Research has shown that those who have developed their brain through education, etc. have a built-in reserve to protect them longer against the storms of dementia/Alzheimers. In other words, if you've built up your brain 20% and then lose 20% to Alzheimer's, you're still okay.
So drag yourself away from the television and get those brain waves oscillating. Use it or lose it...
1) PNAS, April 11, 2000, 97 (8): 4398-4403, "Navigation-related structural change in the hippocampi of taxi drivers"
2) American Journal of Neuroradiology, Pulished ahead of print Oct 5, 2007, "Increased Gray Matter Density in the Parietal Cortex of Mathematicians: A Voxel-Based Morphology Study"
3) Nature Neuroscience, 2002, 5:688 - 694, "Morphology of Heschl's gyrus reflects enhanced activation in the auditory cortex of musicians"
4) Neuroimage, 2002 Nov, 17(3):1613-22, "Voxel-based morphometry reveals increased gray matter density in Broca's area in male symphony orchestra musicians"