The Wall Street Journal provides some tips:

For retirees, not all savings are created equal. When retirees get ready to draw down their nest eggs, few realize that the order in which they tap their accounts influences how much they keep and how much they pay in taxes.

Underlying the tax questions is a fact that many retirees either forget, or don't realize, until it's too late: They pay income tax on money pulled from traditional IRAs and 401(k)s. As a result, for retirees in their 60s, the first stop for withdrawals usually should be taxable accounts, experts say. The thinking: Cash withdrawn from these—despite the name—isn't subject to income tax.

Roth IRAs, which are funded by after-tax contributions, should generally be last in line for withdrawals. Because withdrawals from Roths are made free of both income and capital-gains taxes, it makes sense to allow money to grow as long as possible in those accounts.

So look at your taxable account first, then your IRA or 401k employer plan and finally your Roth IRA.