With your mouth, make a spish, spish, spish sound as you exhale. I'm told it sounds like a baby bird in distress.
There are hundreds of ways to do it - no single one is absolutely correct. The best pishing method is the one that works best for you. A word of caution however - it can be overdone, and is sort of like "crying wolf" if used too much.
The best, and safest, insecticide known to man is bats. Bats are "insectivores" (they eat insects), and a single one can eat up to 2000 flying insects, like mosquitoes, per night. Clearly they have a very high metabolism. For example, the 20-million Mexican Freetail bats from Bracken Cave (shown above) that emerge each night at dusk in central Texas eat more than 200 tons of insects in a single night. No poison is involved!