Cove Molding Holder

Place a screw or nail in the wall, leaving some of it exposed. Hang the cove molding holder on the screw. You want the holder to be hanging at least 1/4" below where you want the cove molding to end up.

Put up 2-3 of these, then hang your cove molding on the holder. Nail one end in place, then slide the molding holder up and off the screw, and take it out. Leave the screw behind.

Fastcap has a variation of these. You can do a Google search and see how they use them.