The coils keep burning up on my lawn mower, which has a 18.5 briggs & stratton motor. How can I keep this from happening and what is causing it?

Hi,
You have not specified a brand, but battery voltage being applied to the ground terminal of the armature is by far the most common cause. Briggs even has a service bulletin on this dating back to 1982 and the most common culprit is the ignition(key) switch.
Peace,
Clarke