Apart from planting companion plants like nettles, which attract the nettle aphid which in turn encourages ladybirds who will devour the common aphid, I found this helps and is harmless to flora and fauna .
Just half fill a spray bottle with warm water,add crushed garlic cloves ( a lot of them as we want it to be really pungent) then add 2 large handfuls of elder leaves to a saucepan of water and cook until they are limp.
Strain the leaves and pour the cooking liquid into the bottle. This works a treat just misting the plants with this solution, the only downside is the stink, when you're down wind