Being well trained on weapons and medical skills is critical, but there are so many others!
Gardening Hunting & processing Sewing Home Repairs Mechanical Repairs Medicinal Herbs Cooking from scratch and off grid Building a fire without lighter or matches (Believe it or not, the unprepared will struggle with this.) DIY cleaning / sanitation candle / lantern making canning HAM radio operator
...and many more!
I've written and edited several for Lisa Bedford. Check out her Skill of the Month at The Survival Mom site
Personally I've increased my skills on: gardening medicinal herbs sewing education (former teacher/current home schooler) DIY cleaning & sanitation off grid living: cooking, cleaning, etc. canning
Add bartering to the skill list; along with a good stockpile of items that will be in high demand during a collapse.