Take a look at: http://www.extension.umn.edu/garden/insects/find/japanese-beetles/
The soapy water works well. Putting a container of soapy water under an area and moving the branch of a plant such as a rose will cause the beetle to drop - right into the water. They always drop if they feel a disturbance. This method, of course, will only work if they are on plants you can do this with.
Hope this helps. Good luck.