Use light cues by purchasing several tiny flashlights. Cover the end of each one in a different color of tissue paper. Assign colors for each effect. From the wings, cue for each sound effect by holding up the appropriate colored light.
Give another style of color cue by using colored fabric scraps the size of handkerchiefs. To signal a sound effect, hold up that effect's color. The more vigorously you wave the fabric, the louder the noise.
Place a large whiteboard in the wings of the stage. Write each effect on the board prior to cue time and raise your hand to indicate the cue.
Connect with the sound crew using headsets. Simple walkie-talkie sets all tuned to the same frequency can keep everyone in constant communication no matter where sound crew members are located in the theater.
Develop hand cues for each type of sound effect, such as one finger raised for a water sound effect, two fingers raised for a thunderclap. Teach all sound crew members to watch for the hand cues throughout the performance.