Just to add onto the "Don't move predictably" portion, this applies both while moving from cover to cover, while fighting the enemy, and while looting. I like to visualize erratic movement as a heart rate monitor. The pulse is steady, spikes, drops, spikes, etc. etc. etc, and you're going to want to move in a similar fashion.
Cover to cover at close to medium ranges, you're going to want to prioritize getting into whatever cover you're gunning for rather than trying to dodge bullets. At close to medium ranges vs. R99s, Alternators, or R301s, the bullet travel time is significant enough that the enemy won't have to compensate for distance too much. At longer ranges, say vs. teams with sniper rifles, moving erratically helps you to avoid getting hit. Not running in a straight line, jumping, sliding, stopping entirely, and even going backwards helps you to avoid those long distance shots, and keep you moving to a better position.
While fighting the enemy at close to medium ranges, I like to visualize a sort of V pattern, in which I move my character model in accordance to the V. The two flat tops of the V, is me standing straight up, and the dip at the bottom is me crouching. Due to the muzzle flash on most weapons, sometimes players can't see you crouch until a few moments after they stop getting damage numbers depending on their reaction time and applied optic. But don't just crouch downwards, strafe left to right, jump up, slide to the left or to the right of the target, keep things moving. Also, don't get caught up on patterns either, keep your movement fresh per target, don't just spam the same, "I'm standing, now I'm crouching, now I'm standing." pattern over and over, because people learn and you will get shot.
Lastly, while looting, don't stand still. Seen way, way, way too many teammates stand still and get downed from long range. Typically, strafing from side to side is all you'll need to do in order to not become an easy target, but keep your ears open for footsteps, gunshots, or anything that will kill you.
Check out this video on advanced movement! I've briefly skimmed through it to make sure it's relevant to the topic of "not moving predictably" and I think it'll help.
https://www.youtube.com/watch?v=TWvNRL3FiN0
