Medicare covers the bulk of your health care expenses after you turn 65. But Medicare’s rules can be confusing and mistakes are costly. If you don't make the right choices to fill in the gaps, you could end up with high premiums and high out-of-pocket costs. Worse, if you miss key deadlines when signing up for Medicare, you could have a coverage gap, miss out on valuable tax breaks, or get stuck with a penalty for the rest of your life.

Here are five common Medicare mistakes you should avoid making:

1. Keeping your Medicare Part D plan on autopilot

Open enrollment for Medicare Part D and Medicare Advantage plans runs from Oct. 15 to Dec. 7 every year, and it's a good time to review all of your options. If you've been prescribed new medications or your drugs have gone generic over the past year, a different plan may now be a better deal for you.

The cost and coverage can vary a lot from year to year — some plans boost premiums more than others, increase your share of the cost of your drugs, add new hurdles before covering your medications, or require you to go to certain pharmacies to get the best rates.

Go to the Medicare Plan Finder to compare all of the plans available in your area during open enrollment. Type in your medications and dosages to see how much you'd pay for premiums plus co-payments for plans in your area.

2. Buying the same Medicare Part D plan as your spouse

There are no spousal discounts for Medicare Part D prescription drug plans, and most spouses don't take the same medications. One plan may have much better coverage for your drugs while another may be better for your spouse's needs.

You should each look up your drugs and dosages using the Medicare Plan Finder to estimate out-of-pocket costs for the plans in your area.

Just be careful if you and your spouse sign up for plans with different preferred pharmacies. Some plans give the best rates only to those who use their network. Consider whether you are willing to go to more than one pharmacy to fill your monthly prescriptions. You could end up paying a lot more if you get your drugs somewhere else.

3. Going out-of-network in your Medicare Advantage plan

If you choose to get your coverage through a private Medicare Advantage plan, you usually need to use the plan's network of doctors, hospitals and pharmacies to get the most coverage at the lowest price.

Some plans only provide out-of-network coverage in an emergency. As with any PPO or HMO, it's important to make sure your doctors, hospitals and other health care providers are covered in your plan from year to year.

If your preferred health care provider is outside of your network, it may be time to change plans or return to original Medicare.

You can switch Medicare Advantage plans each year between Jan. 1 and March 31, during the Medicare Advantage open enrollment period. This is in addition to the annual open enrollment period that runs from Oct. 15 to Dec. 7.

The Medicare Plan Finder can compare copayments, out-of-pocket costs and coinsurance for the plans available in your area. After you've narrowed your list, contact both the insurer and your doctor to make sure they'll be included in the plan's network for the coming year.

4. Not switching Medicare Advantage plans mid year if needed

Medicare Advantage participants have two opportunities to switch MA plans or go back to original Medicare. You’ll also be able to join a separate Medicare Part D drug plan if you return to original Medicare. Your first chance to change your coverage is during the open enrollment period that runs each year from Oct. 15 to Dec. 7. The second opportunity is during the Medicare Advantage open enrollment period, which is held every year from Jan. 1 to March 31.

There are also limited circumstances when you can change your coverage outside of an open enrollment period. Certain life changes, such as moving outside of the plan's service area, can trigger a special enrollment period. And if you have a Medicare Advantage plan in your area with a five-star quality rating, you can switch into that plan anytime during the year. The Medicare Plan Finder can help you search for five-star plans in your area.

5. Not picking the right Medigap plan

If you buy a Medicare supplement plan within six months of enrolling in Medicare Part B, you can get any plan in your area, even if you have a preexisting medical condition. But if you try to switch plans after that, insurers in most states can reject you or charge more because of your health. It's important to pick your plan carefully.

Some states let you switch to certain plans regardless of your health, and some insurers let you switch to another one of their plans without a new medical exam. Find out about your state's rules and the plans available on your state insurance department's website. You can also find more information about Medigap policies in your area at Medicare.gov.

(Donna LeValley is a retirement writer for Kiplinger.com.)

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