If your employer coverage is becoming too expensive in premium, deductible and coinsurance, and you only activated your Part A with Medicare when you turned 65, you can now activate your Part B under a special election period. There are 2 forms that will need to be completed and submitted to Social Security (the administrative arm of Medicare); one form is completed by the employer and the other by you. There is a premium for Part B (outpatient services); the premium is based on the adjusted gross income/IRS tax return. Social Security looks back 2 years to arrive at the premium; they are currently looking at year 2011.
Once you have Part B, you can enroll onto a Medicare supplement and Part D stand-alone drug plan /or/ onto a Medicare Advantage/HMO plan (which typically has drug coverage as part of the plan).
Before making this decision, I work with my new client to review their costs under their employer plan, comparing it to going fully onto Medicare and their choice of Medicare insurance coverage(s).