Before you glue up wood for a project, examine the parts. If at all possible, start out with cleanly cut, perfectly straight boards of the proper thickness. Take out any bow or warp before you begin your gluing work. By using straight and true stock, you won't have to force the boards in one direction or another, and you won't have to get into tricky, complicated clamping set-ups.
What size do you need? Many experienced carpenters use a 16 oz. claw hammer for most jobs, but also carry a 20 or 22 oz. framing hammer for their heavier nailing jobs requiring big spikes. For lighter work, a 13 oz. curved claw hammer can be useful. Even a 7 oz. hammer will come in handy for smaller jobs like making picture frames.