Many digicam flash units put out two flash pulses, a pre-flash to set the white balance and then the main flash. SR Electronics also makes special flash slaves and several different pre-built slave flash units. The MS-1 is a brainless flash and it always puts out the same amount of light whenever it is triggered. Often it can overexpose your pictures if your digicam does not have a manually-controlled aperture that you can stop down. Move the flash units farther away from the subject or use them in bounce mode off of the ceiling or wall.
Indirect lighting is always preferred over direct lighting.
When shooting pictures of people a bounced flash will also limit the occurance of red-eye or glare coming from a pair of eye glasses. Or you can block or diffuse the light coming out of the flash units. Be careful not to block more than a portion of the flash tube though because an intense amount of heat is generatred with each flash and this heat must be dissapated or the flash tube will get damaged. You need your camera's strobe to trigger the slaves but you don't want it to ruin the effect that they create. You can also see what other people have done using the MS-1 and similar slave flash units.