Create a mixture of equal parts thyme, peppermint, clove, and rosemary oils – 4-5 drops of each should suffice. Mix this solution into a small spray bottle filled with water. Shake well and apply to infested plants. This potent mix of essential oils will kill most garden insect pests as well as their eggs and larvae. This combination also works great as a general purpose outdoor / indoor insect repellant!
Another plan to avoid aphids is to plant onions and garlic around your garden. Aphids dislike the organic compounds which give onions and garlic their signature aroma. Scallions, leeks, shallots and chives are in the same family with onions and garlic and should also work.
Remember some of these oils are hot. Never get essential oils in the eyes, if you do use a vegetable oil to dilute. Water will not help and just drive the oil deeper.