A high pressure water jet is good. Use a pump action sprayer so that you can build up pressure and then have one hand free to turn over the leaves and spray the underside. Squish plenty by hand – it also makes you feel better – the aroma of aphid corpses will attract more predators like lady bugs and lacewings to help with control.
Insecticidal soap can be a next-level control, but then you don’t get so much help from the predators.
