Just because you own an adventure bike, that doesn't mean you're ready to go adventuring on anything gnarlier than your average gravel road.
The truth is, most new ADV machines don't come from the factory ready to rock-n-roll in the rough stuff. I don't have years of off-road riding experience and I haven't crossed international borders to kick enduro ass, like Spurgie, but as a longtime wrench, I do have some knowledge about parts that can make your ride better and setup techniques that can help you ride faster and have more fun.
Stuff to buy
If you’re going off-road, everything is your enemy. Trees, rocks, sand, and dirt all conspire to hurt you or your machine. You obviously need a lot of gear for yourself, but your bike needs some protection, too. Spurgie wrote a pretty thorough article on selecting a set of crash bars, and I’d say those are pretty important — they might be the best piece of protection in terms of cost-of-purchase vs. cost-of-not-purchasing. Note that aftermarket crash bars often provide a lot more protection than the skimpy OEM ones that might have come with your bike.
Another necessity is a skid plate. Yes, your bike may have come with one on it. It’s perfectly adequate for baking a batch of cookies. That’s about it. Having your cases or oil pan out in the open is an invitation for disaster. Skid plates cost about what a tow and 12-pack of good beer will run you, and you’ll need both after a crash if you don’t buy one.
Also, budget for a set of tires. If you’ve got adventure bike-sized wheels (17-inch rear and 21-inch front), put some serious knobs on them. There are a multitude of tires in ADV sizes, and almost all of them will perform better than the crappy street-biased skins that came on your bike.
Your final piece of necessary protection is a set of handguards. Levers are fragile things, and so are hands. Your bike might have come with some wimpy handguards, but they probably don’t have a frame. A set of aftermarket handguards with full aluminum frames will help keep your hands from getting hurt, and they’ll also drastically reduce the amount of damage a tipover inflicts upon your bike, especially the levers.
I feel like those are the minimum required items to safely take your ADV bike off-road without breaking things. Your mileage may vary, but you can spend money on protection pieces, or replacement parts. Don’t forget, too, that if you’re deep in rough stuff and your bike can’t move under its own power, extracting it could be a bit of an ordeal.
Stuff to do
ADV bikes are versatile creatures, but you can make some setup adjustments to them to make riding them off-road a bit easier. The easiest and most beneficial change you can make is to pull off your panniers, if possible. The bike will be slimmer and lighter. It’s not always a possibility, but if you’re camping near where you are riding and can secure your gear, it can make your off-road excursion a bit easier.
Another worthwhile change you can make is to air down your tires! Many ADV bikes spec 42 psi in the rear and 35 psi at the front, but that’s for street use. Less air pressure will enable traction-producing tire deformation. How far down do you go? Well, that’s a matter of preference and opinion. The higher your pressure, the less risk you have of getting a pinch flat or slipping the tire off the bead. You also have a lower risk of dinging a wheel. (This is a matter of great importance to those of you who do not have spoked wheels!) If you’re coming from a dirt bike background, remember that these bikes are way heavier than a dirt bike. Eight psi might work fine in the rear tire of your lightweight trail bike, but that’s really too low for an adventure bike. For most riders, 15 psi to 25 psi seems to be the sweet spot. When you return to pavement, don't forget to air back up or ride extra carefully until you can!
You might also want to consider your riding position on the bike. ADV bikes are set up to be comfy on the road, where riders are seated. If you’re hitting the trails, though, you’re going to be standing most of the time. That means your angle of attack on the controls will be a little different. Rotating the handlebars forward in the risers a bit will help you get a more natural bend to your wrists, minimizing fatigue. Two-fingering a clutch is pretty hard when your hands hurt.
Your foot controls also need a little attention. Pull your footpeg rubbers out so your boots can bite into the serrations on the peg itself. If you have a brake pedal that can be adjusted, this can also yield a more comfortable interaction with your controls. Raising the brake pedal just a hair above level with the footpeg is a good starting point. (Keep in mind that pulling your footpeg rubbers has the effect of dropping you slightly on the pegs, “raising” the brake lever relative to the top of the footpeg. You may find your brake pedal is just right when you're standing on the pegs as opposed to the rubbers.)
The final tweak you can make quickly is to adjust your suspension. If you’re switching from two-up to solo riding, or skipping the panniers, you can probably safely back off your preload a little because your “sag” will change. Suspension adjustments is a complex topic and there are folks far more qualified than I am to tell you how to make them, but setting the sag based on the weight you are carrying is always the starting point.
Taking a few hours in the shop and a few minutes before you hit the trails can yield a less stressful ride. If you’re feeling fresh and your bike is undamaged, you’ve got the two most important ingredients to spending a day happily spent off-road.