Aaron's (me) rule of 3 - always have 3 ways to solve a problem and 3x the amount of supplies that you need. I do Search and Rescue, and have 3 days food/fuel even though the longest mission will be 18 hours (by law).... so here's how it plays out.
Always have at least 3 ways to solve a problem. For example - you're stuck, without another vehicle to remove you. You have 1) a hi-lift jack and tools to properly use it, 2) a shovel, and 3) good tennis shoes to walk home.
How I learned this rule.
I went wheeling in the snow and got stuck. I had no shovel, no base for my hi lift jack, and moon boots. I can attest that you can walk 20 miles in moon boots - but having decent shoes would have saved me a substantial recovery time.
Today, if I were to head to the hills. I have, a well-maintained 4x4 with lockers front and rear, winch, gas cans, mechanic tools, tow rope, snatch strap, high lift jack, gloves, shovels (snow, dirt), chains, logging chains, food for 3 days (plus a day of snacks), gear to survive to 0 degrees, several fire starters, stove, GPS, batteries, flashlights, water... lots of water, ways to make water pure, trekking poles, well used (but not worn out) hiking boots, gaters, wet-weather gear, chem lights, axes, saw, radios (HAM, CB, FRS/GMS), shelter for outside the vehicle, extra clothes, socks, first aid gear (medical responder), 1st aid training, survival training, responder (not EMS, just slightly less) training, a gun, Bible, several knives, hat, long underwear... and I'm sure I'm forgetting some of what's there. Point is, I never ever want to be so close to killing myself again through my own stupidity. I also have 30 years of experience from the date that happened.