When you think about the expenses in retirement that might bust your budget, healthcare is probably one of them. Even though Medicare Part A is free for most enrollees, there are premiums associated with Part B. And while some Part D and Medicare Advantage plans offer $0 premiums, those may not be the optimal plans for you.

Even if you're able to keep your Medicare premiums to a minimum, there are still deductibles, copays, and coinsurance to think about, not to mention services Medicare doesn't cover at all. So it's important to do what you can to keep your healthcare costs more manageable. And part of that boils down to knowing these three key Medicare rules.

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1. Enrolling on time could save you money

Your initial Medicare enrollment window starts three months before the month you turn 65 and ends three months after that month. If you miss that window and don't qualify for a special enrollment period, you could end up paying more for Medicare Part B for life.

You'll actually face a 10% surcharge on your Part B premiums per 12-month period you could've had coverage but didn't sign up. And going too long without prescription drug coverage could lead to higher Part D costs, too. So make sure you know when you're supposed to enroll.

2. Prior authorization protects you

One drawback of Medicare Advantage plans is that many require prior authorization for a number of services and diagnostic tests. But if you don't get that authorization when you're supposed to, you could run into trouble.

If you're not sure if a given service requires prior authorization, contact your plan's customer service line and ask. Don't just assume your providers will submit the right authorization requests.

3. Switching plans is an option every year

You might think that once you choose a Medicare Advantage or Part D plan, you're stuck with that same coverage for life. But thanks to Medicare's open enrollment period, which runs each year from Oct. 15 through Dec. 7, all enrollees have an opportunity to make changes to their coverage.

During open enrollment, you can switch Part D plans, get a new Advantage plan, or drop Medicare Advantage and sign up for original Medicare instead. It's worth reviewing your plan options every year, even if there's nothing obviously wrong with your coverage. You never know when a different plan might offer key savings.

It's important to budget carefully for healthcare costs in retirement. But it's equally important to know how Medicare works, and how your specific plan works. Doing that legwork could help you save money in the course of taking care of your health.