Strong streams of plain water is an effective method to dislodge aphids. The safest (environmentally and for your plants) product to use would probably be a commercial insecticidal soap spray. House detergents can cause significant plant damage (as folks have posted right here in the gardenweb), without doing the job intended on the aphids.

smms,
I grate about a tablespoon of Fels-Naptha bar soap to two gallons of water. Not damaged anything. But just like rhizo says,"Strong streams of plain water..." do just as well for aphids and far less work.