The best way to prevent Fungus gnats is to pre-treat all soil from the beginning with BTi before any hatch. BTi will only work on larvae, so if you have fliers, they will quickly overwhelm any IPM that is based solely on BTi alone.
If you find yourself with fliers and larvae start using BTi (gnatrol 28%, Dunks 10%, Microbelift 9%, bits 3%) I would do a full drench for a few days, then let it dry out as best as you can.
Get yourself an inexpensive and low power shop vac ($20 you don't want it to damage any leaves if you get too close) and give the stalk a shake and get around the base. This also works easiest if you have white walls. Stirs up the top layer of your soil to dry it out quicker.
BTi drench again and they should be on their way out. Vacuum the fliers as often as you can. The larger the size the closer to it giving you more.