Is it a banner sign that can be rolled up and put in a tube mailer or a hard 3 dimensional sign like on the buildings of stores?
If it can be rolled up, then priority mail has these triangular mailing boxes that work well.
If it's a large solid sign and can be broken down into smaller pieces then I'd use a lot of bubble wrap or those foam bags and ship them in smaller boxes.
If it can't be broken down I would put it on a pallet or make a custom crate (there's also companies all over that do this service) then the crate can be filled with various packing supplies and ship it freight. If you use a crate or pallet then I'd make sure the sign is strapped down inside to the base of the crate/pallet so it doesn't shift.
