Perhaps you've never thought of "buying" your retirement. Yet that is exactly what you do when you put money into a retirement nest egg. You are paying today for the cost of your retirement tomorrow.The cost of those future years is getting more expensive for most Americans, for two reasons. First, we live longer after we retire - with many of us spending 15, 25, even 30 years in retirement - and we are more active.Second, you may have to shoulder...