I always have those darned gnats - no matter what I do it seems. But - I have noticed that they really are not doing any harm.

That being said, I have read that keeping the soil very dry on the surface will reduce their reproduction and also yellow sticky cards, placed standing up on the soil surface, attract and hold the adult fungus gnats and give you an idea of the size of the population (I know for sure they are available at Lee Valley).

Another tip is: Placing a slice of potato on the soil surface sometimes attracts the feeding larva. The potato slices can be used to collect and dispose of larva and to gauge when the larva are actively feeding, for timing of pesticide applications. Make sure the potato slices do not dry out.

Here is a link that might be useful: Yellow Sticky Traps