This may be a silly question but is there any soil treatment I can begin either in the fall or spring to prevent aphids on next year's tomato crop?
It's not a silly question, but the answer is "no". Aphids do not reside in the soil, nor do they lay eggs in soil. The best thing you can do is maintain a healthy soil system by top dressing with active compost (or other organic matter) occasionally. Next year, avoid excessive nitrogen that might encourage excessive vegetative growth...always an aphid magnet.
Thank you and yes I did learn the excessive nitrogen lesson this year with my veggies.
Or a blast from the hose with wash them off.