I understand that plums & cherries are just very susceptible to aphids. Last year we had a VERY bad infestation on ours, especially the plums. Is there something we should do now, while they're dormant to help fight off aphids this year?
pruning them makes them less warm and humid and therefore less susceptible to both disease and bugs.
In the long run, having diverse and native plants will attract a variety of helpful bugs, so nothing gets out of control. Having a diversity of animal life also helps with that. Going organic and using mulches is a great way to do both of those.
John S
PDX OR
Aphids can be a problem at times and with large trees methods of control are more limited than for smaller shrubs or flowers.
1. spraying with a firm water stream
2. Using insecticidal soap
3. insecticide
Remember almost anything you use to kill the aphids will also kill other insects. The goal here is not to have to use excessive inputs weather you are organic or not.
The use of predators is a good strategy and here is some food for thought.
1. Buying and releasing Lady Bugs -- most of them just fly away.
2. using native plants to encourage the presents of predators works well most of the time.
3. Tolerance, predators need a food supply too.
Randy
Yamhill County Master Gardener
I have cured my aphid problem. Look on Google images and see the ants sucking the sweet juices from the aphids butts. The ants herd the aphids to the very ends of the branches last leaves and prevent them from traveling back up the branch. They then 'milk' the aphids. Come fall they carry the aphids down into the ground and keep them there until spring, whereupon the cycle repeats when they bring them back up into the tree. My cure is to wrap a nylon stocking around the base of the tree and secure it with a safety pin. Every spring, I renew coatings of sticky Tanglefoot on the stocking and the ants get trapped. They will even build grass blade bridges over the sticky stuff to cross over and get the aphids back working. Go out on a warm night and watch the ants working.
When you take a sweet cherry that is infested with aphids at the ends of its branches, and install a stocking -Tanglefoot barrier, in a matter of days it comes back to life. You can see its new vigor! Be careful, the ants will find ways to climb up between bark furrows and valleys. They will even climb neighboring trees and cross over to the protected tree! Don't apply much Tanglefoot directly on the bark except in places the stocking does not reach like deep valleys. 
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