Sand-mulch. Just be aware that sand can be really alkaline, and burn plants trapped in pots? Maybe rinse?
Check your drain holes at the bottom. Are they shady little Gnat Condo's? I have had issues with the drain rock at the bottom of my pots - it lets gnats -and fruit flies access roots, bits of compost, and moisture, at the shady bottoms of many pots.
Old nylons placed into the pots before soil can help block the little buggers from flying in, as long as they don't plug up and cause water logging.