ceiling joist storage expression
First you need to determine the weight of the items you plan to store. If the trusses were designed under the IBC/IRC or the UBC (and maybe the BOCA and SBCCI) codes, then they should of been designed with a 10 psf live load on the bottom chord. If you keep below this loading you should be okay, but you might want to check out the code requirements for your building. 1/2" plywood will add a 1.5 psf load. A 2x10 at 24" o.c. will add 2 psf (but a 2x4 will add .75 psf) to the ceiling. A 2x12 at 24" o.c. will add 2.5 psf. Subtract the plywood and add-on from the 10 psf to get what load you can add. The safe bet is to consult with a structural engineer familiar with your truss assembly. Could prove to be the lesser alternative to a truss collapse.