Having as many of their drugs as possible included in a plan's formulary, even though the list changes every year, is the single most important factor in choosing a plan. If more than one plan with all drugs on the formulary is available, choose the plan that has the fewest restrictions on access to those drugs and the lowest total costs (not just the lowest monthly premium). Check whether the plan waives any of the deductible, and total up the copayments they'd pay per prescription. If they spend a lot on medications, also consider whether there's any coverage within the "doughnut hole." Finally, check to see whether the pharmacy they prefer participates in that plan.

A plan that at first seems the right one may turn out not to be the best. It may have restrictions that didn't seem especially important when they signed up for it, but have since proven to be a problem. The plan might change its formulary more than most from one year to the next, change its rules or restrictions, or raise its premiums. Or maybe a new plan is now offered that has better terms. In any of these situations, a person can leave his current plan and enroll in a new one. But he must do so by signing up during the open enrollment period from November 15 to December 31 each year.