One would be physical fitness. A lot of people think of it more in terms for self defence but life during shtf will be hard and people will have to work hard to stay alive. Being in shape will make the hardship easier. I would say another good skill would be redneck engineering. Lots of things will no longer work the way they used to. Repurposing or retro fitting will be a good skill. To some it is second nature but others need to study and get some ideas.