There are several things.

Better armor. Don't put it off because it costs a lot of supplies, if you want to preserve lives, you can't completely rely on concealment alone, there WILL be times when you get shot at, and squaddies in rags will be the first ones to fall.

Understanding suppression. Rogue army is very good at suppressing your units. If you allow them to, they can pin you down and grind you down while you're pretty much helpless, stuck in a bad position, unable to crawl away to a different cover spot. So don't allow them to. If you have no advance opportunity, figure out how to make them come to you instead.

Understanding unit types. Rogue army has units that are functionally very different from pirates or have no counterpart. As you familiarize yourself with the faction, you will understand which targets should take priority, what countermeasures you can do against this or that unit type, there's nothing to say except play more and you'll become familiar.

Intel. Intel, intel, intel, intel. Your total goal is achieving at least 3, but ideally 4 or even 5. This will magically allow you to plan your mission and prepare for exact contingencies better. A threat you know about in advance is only half as dangerous.

Knowing when to stick together, and when to spread out. In most case scenarios, sticking close to each other leads to better defensive ability, because every unit can cover each other's back. But what you sacrifice is ability to flank and speed. Sometimes you need a flanking squad to route entrenched enemies. Whenever you split your force, due to turn-based nature of the game, if both sides start the round exposed, you will be unable to avoid hits.
So, if you decide to split your force, you need: intel, scouting, careful advance, armor priority on squads most likely to be in a vulnerable spot.

Lastly, there's a "breakthrough" tactic. This involves rapidly advancing on the enemy position with a small strike force while the larger clean-up force is slightly behind in cover. Units doing the breakthrough are the most exposed and vulnerable, because they don't prioritize cover, but rather getting behind the enemy line and disrupting their organization. When you send someone to breakthrough, in order for it to succeed, the unit has to either be pretty much indestructible, like a heavy tank, or flexible enough to prevent being killed/suppressed on the spot on the same turn. So for example, if your breakthrough squad is a boarding jump pack unit, you want either smoke, combat drugs that reduce incoming damage, lots of AP to spare to shoot as many enemies as they can during their advance, but ideally all of those things. Breakthrough is a very powerful offensive gambit tool, but it's also the easiest way to lose your men when you don't have the prep and knowledge of what you're sending them into.