How should I hang racks from the garage ceiling with trusses?
2x's laying flat as opposed to on edge are going to bend or break when weight is suspended from it over time. The longer the board the faster it will compress or fail. To remedy this problem you must add support to the existing framing members. I think the simplest way is to install additional 2 x 4's (on edge) attached against the edge of the original 2 x 4's. It should have a "L" shape profile. You didn't mention the span of the rafters, but you may consider adding webbing above the platform. All this is are lengths of 2 x 4 attached to the platform joists at one end and the other to the mating roof rafter above. This is probably over kill, but still another way to prevent a failure.