How to avoid your precious tomatoes from being given a nasty suck by these little pests is a problem.
You could stay up all night with a torch and catch everyone of them but that would be a bit extreme! Instead, my method is to distract them with an old damaged tomato in an easy place for them to find.
Another thing they love is a French Marigold. Growing these between your plants (companion planting) is supposed to deter some aphids - but slugs and snails love them and I would rather sacrifice a Marigold or two than my best tomatoes.
If you go out into the garden with a torch just after sundown, you'll find most of these pests and be able to throw them into an area of your garden where some other little animal such as a hedgehog may find a meal. I sometimes hear them crunching in the night!