I used to have a much bigger aphid problem than I do now. It's so much better, I believe because of a a couple of factors.
First, I don't fertilize with store-bought nitrogen anymore, so there isn't a big flush of lush new tender growth all at once that the aphids go crazy for. The trees and other plants grow slower, but they don't offer as lush a target for the little sap suckers.
Second, I have a pretty extensive poly-culture of supportive plants throughout the garden, including lots of flowering plants, herbs, bulbs, ground-covers, bio-mass producers (comfrey), insect repellent plants (tansy is my current favorite), and veggies, veggies, veggies. I don't plant anything in a row -- everything is planted in little drifts (carrots, beets, lettuce, okra, etc.) or in single plants where there is a window of space (peppers, tomatoes, various vines). The result is a massive poly culture that encourages an equally massive insect community, from the tops of my tallest trees (avocados) to the surface of the soil and even below. If you want aphid killers to populate your garden (lady bthirugs, spiders, praying mantis'), then you've got to build habitat that they want to live in.
Third, because aphids reproduce so rapidly, while lady buds and other good insects are much slower in their reproductive cycle, what ever you use to kill the aphid may have a much greater harmful impact upon your good bugs. If you use something like neem oil or tobacco juice, it isn't just hard on the aphids, but also the spiders and good guys.
It took me about 4 years once I stopped using any bug sprays (even natural ones) to see the system come into balance. There are still times where I see an infestation of one sort or another, but generally, because things are so spread out and not planted all in one spot, even if the aphids get one area, they won't get the others.
Slugs, on the other hand, are the bane of my existence. I've got a great lizard population doing their part, but once the slugs find the cabbage, it's hard to
root them all out and save the plants.
Best of luck.