let the soil dry out between waterings they do like damp soil. I have used the sticky fly traps in the past but just ignore them now.

I had the same problem. I simply have put few drops of Fairy liquid in the water I use for watering my houseplants and it did a trick. I'd say repeat it after 7-14 days.

They don't really do any harm,so just follow either of the advice given above..

With plants more sensitive to drought or soap, such as ferns, you could also use a product containing Bacillus thuringiensis israelensis, often sold for mosquito control.
Also pour chlorine bleach down any open drain, since the gnats will also feed on fungus in the drains.

Did you pot the plants up with your garden potting compost if so that could be your problem, as it happened to me, best to pot up in Houseplant compost, I did get rid of mine by spraying around them with house fly killer, it didn't harm the plants either, I did it every day till there were no more. amazing how many little flies there were, covered my light topped book case.
Hope this has helped. :o))