Vice President of the Center for Medicine in the Public Interest.

From now until Dec. 31, Medicare Part D is offering an “open-enrollment” period, meaning that seniors can change their prescription drug coverage to pick a new plan that better meets their needs.

Seniors should jump at this once-a-year opportunity. Instead of being forced into a one-size-fits-all plan run by the government, Part D participants can choose among a variety of private insurance plans, custom tailoring their coverage to their particular drug needs.

Indeed, the Centers for Medicare and Medicaid (CMS) estimates that the average monthly premium for standard Part D coverage in 2009 will be $28, only $3 higher than last year, and a whopping 37 percent lower than the $44.12 monthly premium projected for 2009 when Part D was launched.

The prospect of substantial cost savings is another big reason why savvy seniors should sign up for Part D. “Until I enrolled in Medicare Part D I paid more than $4,000 a year just in premiums,” says Joan Wells of East Aurora, New York. “Now I only pay about $300 a year for coverage.”

You don’t have to be a rocket scientist to pick the right plan. Simply make a list of all your current prescriptions and dosages. Then visit Plan Finder, an easy-to-use, interactive comparison tool at Medicare.gov. Once you enter your information, Plan Finder kicks out a list of all the appropriate plans and their costs in your area.

In the coming weeks, seniors should take advantage of the open enrollment period and sign up or switch to a new plan that better meets their needs.

Seniors should also pay attention to any effort Congress makes to take those choices away. Many lawmakers are pushing for a redesign of Part D, as they’d like to turn it into a more traditional government program with just a single, price-controlled plan. By taking the opposite approach, Medicare Part D leverages market forces to create competition, leading to lower prices, better service, and more choices. And choice is what better health is all about.