Thanks for writing. In most cases, when you knock aphids off a plant, they won't climb back on. But because there may be aphid eggs on your lilies, you'll want to watch them and be prepared to spray with water again a second time if you see any of them hatch and begin to grow.
If you'd like a more natural product, try a spray such as insecticidal soap, Captain Jack's Deadbug Brew, or a product with neem oil. These are all relatively low-impact products.
By the way: We often get asked for home-made remedies for controlling pests. Some of the "recipes" I've seen are actually more dangerous than the products you buy at the store. Just because you use a product (such as ammonia) at home doesn't mean it's safe.
---Justin, Senior Garden Editor, BHG.com