Hi Marilyn! Two ideas:
1. use a diluted soapy water spray on the plant, using a mild, natural soap of course -- do what you can to try
not to get soap in the soil of a potted plant (not as much of an issue with outdoor, non-potted plants). Be sure to get the underside of the leaves. Soap removes the waxy coating on the aphids which dehydrates them and then they die. Usually soap doesn't bother the plant, but without rain, it might be good to rinse the plant a day or two after the aphids die off. Definitely not a good idea on some types of plants, which is probably why "soil" asked what type of plant.
2. slide them off with thumb and forefinger and squish them (or collect them and wash them down the drain if you're squeamish). Safest for soil and plant, but might have to do daily until you get them all, or might be hard if they're hiding in stem and leaf joints, etc. Similar to Leah's idea, except more up close and personal than a blast of water.

You know, you could possibly do a water blast in the shower. Again, depending on the plant.