I do the same trick Carlos does except I use juice with a bit of dish soap in it

It also helps if you feed your frogs in the same place every day. They will eventually learn where to go to get food and will help them clean up the flies as soon as you put them in.

My Tincs devour everything I give them, but my Mantella and Vents eat when the mood hits them so I have to be careful I don't over feed those tanks.