A layer of perlite may work well for these. It stops them thinking the top of the soil is moist from what I remember. Also dunk watering helps as the time between watering is longer and therefore the top dries out for longer periods.
I had a major problem with both fungus gnats and aphids last year
Fungus gnats i fixed with an extra layer of perlite on the top and the aphids were sorted out by some online Ladybirds which decided to stay in my greenhouse until everyone of the little ******* had been eaten