Next would be a good filter. Merv 11 is a good option. Merv 11 has a high filtration rating without putting to much restriction on airflow. Merv 13 FIlters tend to increase the static pressure with-in the system, be sure to have a technician evaluate static pressures if your going with a Merv 13. To much restriction on airflow can lead early blower motor failure.

 Smart Filters are a good option, just make sure to check your reminders.