First of all, I guess you're using a iron rod (core) or something around which you are wounding the coil. (Not shown in pic).
Now, reversing the polarity will cause the electromagnet only to change the magnetic orientation ie N-pole to be S-pole and vice-versa. (considering you are not using both the batteries together)
To increase attractive power increase current in coil, and of course diammeter of coil wires should support that higher currents to avoid burnout of wires.