The front springs rarely go bad unless serious rust has set in. The rears can sag if the car has been overloaded or again, rust has set in to weaken them. The first thing you should do is replace the shocks, I think this is the majority of your issue. You can also add an anti-sway bar up front if you want flat cornering. If the rear leaf springs are a LITTLE saggy and rust is not the issue, I have added rear trailer towing "helper springs" with great results on the cheap. Pep Boys has them.
71HANTO