Avoid excess microwaving plastic containers. And don't keep old plastic wares for too long. Change them periodically (annual for permanent wares, 2 months for disposable wares).

I only microwave 30 sec, frown on microwaving more than a minute. My wife microwaves for 2 minutes or more! The latter is a problem because I know (some of) the food gets unevenly hot (while the other parts are still cold) that it melts the plastic.