You can start saving extra right now, earmarking that money specifically for future health costs. Fidelity Investments ran the numbers and figured out that a 65-year-old couple who retired in 2009 will need $240,000 of their own savings to handle 20 years of retirement health costs such as Medicare premiums, out-of-pocket costs, and prescription drugs.
You can stay as healthy as you can, keeping your weight down and quitting smoking, for example. Healthier people often get charged smaller premiums.
If your employer lays you off or offers you an early retirement package before you turn 65 (when Medicare kicks in), negotiate hard to keep your health benefits as long as possible. It's easy to get worked up about getting more pay added to your package, but extended health coverage can be an even more lucrative benefit. So try to get that if you can.