I just realized I posted this in the IPM section when I meant to post in the pest & diseases section... Reposting there.

Answer is still the same. Remain alert. And when you find them, squish or wash off.

Aphids are generally more likely to infest plants with lush green growth, plants growing in soils with too much available Nitrogen although they can be found on other plants. Aphids also produce live young, and start doing that almost as soon as the are born so by the time you see an Aphid you probably have several generations already present.
The simplest means of control is to knock the wee buggers off the plants with a sharp spray of water. Now many people that know not whereof they speak will tell you that those Aphids you just knocked off will simply return, but these wee buggers are so dumb they know not whereof they came and die before they figure out where they need to go. What you will see is the next generations moving into the vacancy you created, so it takes some time and repeat applications of the water spray, as well as digging into your soil to determine why your plants might be so attrractive to these pests.

All true above. Don't think you're going to garden without aphids, because you can't. Squish 'em or knock 'em off with a spray of hose water.

I tell you IMO that the lady bugs I have are having a field day munching the aphids on my plants!!! I used to have them everywhere seemingly overnight. Then I released some and I have ladybugs hanging round for weeks now munching! Most did fly away but my aphid problem is now very minimal!

People that have continuing problems with insect pests really need to take a good look at the soil their plants grow in because plants growing in a good, healthy soil will be much less attractive to insect pests and you will see fewer of them. I see very few Aphids or many other insect pests on my plants which grow and blossom prodigously. When weather coditions are right I may need to spray to control Powdery Mildew on the Lilacs, but then i am not very diligent about the soil they grow in.

The rose bushes I have in my perennial garden become completely decimated by black spot and aphids by the end of the season. I've had the soil tested every year, fertilize, mulch, weed, etc. Everything else around them does great but it truly is a constant struggle. This year I planted some lavender b/w the plants and found a fabulous homeopathic spray that has been working like a charm on the black spot (plus I've been very diligent in picking off any suspect leaves so they don't fall & pollute the ground around it) AND I bought a canister of lady bugs like OrganicIslandFarmer. I haven't seen one leaf with holes. I see the ladybugs flying around everywhere now so they must be happy! Which makes my roses very happy. And even though it's still early in the season - this is the best they've ever looked even at this early point! Go get you some ladybugs!! :-O

When I spot aphids, I go to the local nursery and buy lady bugs. My oak tree and the roses are the first targets for aphids, so the ladybugs get applied there. That's it for the season - aphid problem solved.