When you retire, you will probably still need at least 85% of the income you take in now. Yet if you also have to pay retiree medical costs and are depending on only your 401(k) and Social Security, you will be left with about 57% of your preretirement income, according to a study by the benefits firm Hewitt Associates. To boost your nest egg, don't retire until 67, the age at which many workers are eligible for full Social Security benefits, and contribute an additional 2% of savings to your 401(k) plan.