The most obvious way is to rethink your standard of living in retirement. Swapping the around-the-world sailing trip for a Caribbean cruise may help you lower your retirement target to a more attainable goal.
You can also delay your planned retirement date from, say, 62 to 68 or so. Working past the traditional retirement age will let you postpone withdrawals from your retirement accounts. Your savings will have more time to grow, and you'll reduce the number of years you'll need to draw on them. Working longer may also let you delay taking Social Security until you reach at least full retirement age (66 if you're 50 today), potentially increasing the size of your monthly benefit by 30% or more.
The great thing about online retirement savings calculators is that you can play with the numbers to see exactly how much more or less you'll need to save based on when you plan to stop working, or how much you'll spend in retirement, or any number of other factors.
Working part-time can help too. But the problem is that you don't know if you'll have the interest or energy to work at an advanced age - or if you'll have health problems that prevent it. You also may have a tough time finding an employer who wants to hire you in your later years for the amount of money you want to earn. So pinning your entire retirement strategy to working in your 70s or beyond isn't such a great idea.