Aphids usually appear as the weather begins to warm; they will first be noticed on plant tips, near opening flower and leaf buds. Aphid populations tend to be higher in plants that are fertilized liberally with nitrogen and are heavily watered, as these practices produce flushes of succulent growth. Avoid excessive watering, and use slow-release or organic sources of nitrogen. Properly fed plants tend to be less troubled by aphids. Apply thick mulch in the summer to keep soil cool and moist.
Heavy rain, especially when accompanied by strong winds, can significantly, reduce aphid populations by washing the aphids off the plants. Crops grown under sprinkler irrigation may also have fewer problems with aphids.
It is a good practice to monitor susceptible plants, and then at first sight of aphids simply wash off the plants with a stiff jet of water or by hand wiping. Cradle the plant tips in your hand as you wash off the pests. Sticky honeydew and sooty mold can be washed off with a mild soap and water rinse. Adult aphids can also be killed by spraying plants with insecticidal soap, pyrethrin, or with the botanical insecticide neem. Take care to apply these treatments to the stems and the undersides of leaves. 
Aphids have many natural enemies that can be effective in reducing aphid infestations including lady beetles, syrphid fly larvae, green lacewings and parasitoid wasps. Look closely for evidence of predators and avoid broad spectrum insecticide applications that would disrupt these natural enemy populations. Braconid wasps are especially important parasitoids. Lady beetle adults and larvae eat as many as 200 aphids per day and as many as 2400 in their lifespan.
Aphids are attracted to the color yellow, and this can be used against them. Bright yellow plastic dishpans make a good trap when filled with soapy water and set near infested or vulnerable plants.
In summary, try combining many methods in your approach to controlling these pesky insects, such as the biological controls, physical/mechanical controls, and cultural controls discussed here.