For sheeting, or a series of ribs, using long sticks with a few weights works very well. Another idea for sheeting is to use old telephone books, which conform nicely to a wing section and don't concentrate the load unnecessarily. For sheeting around a cylindrical shape like a fuselage, I use ace bandages. They nicely distribute the load - avoiding uneven pressure. You can wet a sheet of balsa, slap it over a tube or pipe (as a preliminary form), wrap on the ace bandage - and it will dry quickly. Then the balsa is very close to the final shape when you want to glue it to your formers.