What did you do for chassis stiffening?

As the title asks, what did you do to stiffen your car? Frame connectors, core support, torque boxes, front frame braces? Did you make them or buy kits like the us car tool stuff? I'm just getting ideas for my Duster and would like to hear and see what you share.
I'm toying with making frame connectors, and thought about the core support as well and want to educate myself and see what you all have done.