I've got a fair bit to say on this, most of it learned the hard way or due to a situation that was difficult to get out of. Here are a few things to start with.
1 - some 2x4s or something to support your jack on a soft dirt road
2 - a good tool kit with sockets and wrenches that fit your vehicle SAE/Metric (not just for overlanding)
3 - a shovel
4 - jumper cables (not just for overlanding)
5 - a winch
6 - soft shackles
7 - tree saver straps or recovery straps
For the recovery gear, make sure it's rated high enough to support your vehicle - working loads and breaking strength are two different things
8 - an old Army BDU or ACU jacket or something you can wear when you need to roll around the ground
9 - first aid kit - that is readily accessible
10 - fire extinguisher - that is readily accessible
11 - air compressor for airing up your tires
12 - heavy work gloves
13 - extra food
14 - extra water (1 gallon per person per day that you might be stranded) and don't forget that the people you encounter on the trail might be out of water too. At Big Bend in December in my Jeep, I came across two mountain bikers that were miles from nowhere and out of water - they were very appreciative of the water.
15 - GMRS Radio
16 - Paper Maps of the area you're going to, shockingly, GPS doesn't always work and also, it's really difficult to gather a group of people around a screen to plan the route. And satellite navigation is pretty useless for terrain association. Especially when roads either do exist or don't exist on the Nav.
Not something you can buy but:
- Patience
- Training / Education
- A Good Attitude
- Being Open to Learning and realizing that you don't know everything and maybe what you think you know might not be right in this scenario.
The difference between Training and Education is like this:
Education - I know that First Aid is important and I have a First Aid kit that is well equipped.
Training - I've opened my First Aid kit and really used it or at least simulated use in various scenarios.
Likewise, I have a fire extinguisher but I haven't checked it in about 3 years. I assume it's OK but I should find out.
I'm sure I'll think of a few more but that should get you started.