The settle-in-time is huge IMO. I wait until the animal seems comfortable and active and curious before I start handling them.
My new babies get a bit defensive when I open their bins right now. So once I open it I let them see that it's me and I wait for them to calm down and get curious before approaching. Anytime they get defensive, I stop and wait. If an animal is aggressive, I block them (back of hand, side of water bottle, anything they can't grab to bite) until they stop and calm down (repeat as needed). This even works on my Psycho (at least to the extent where it distracts him enough I can catch him).
Some reference:
Fatal_S wrote:- Curious - Looks interested. Tongue flicking. May slowly approach.
----- Allow them to approach and check you out. Move slowly and make sure they can see you approaching. You can try gently touching them and picking them up (make sure to support their legs quickly). You can offer snacks or take them somewhere to explore.
- Explorative - Moving around. Tongue flicking. Tasting things. May pee/poop from being active. May test-bite you/objects.
----- Let them explore secure areas or closely supervise them. They're apt to climb things, fall from things, and get into places they shouldn't be. You can let them try to bite things providing it's not dangerous. I let my babies test-bite me and they seem to realize I'm not good food. I usually block my adults from test-bites now.
- Defensive - Body puffs up. Body held sideways (shielding for protection). Flaring tongue out. Mouth gaping. May "hiss" with mouth open.
----- Wait. You can move around slowly and/or make some noise (I gently drum my fingers on the ground) but don't approach. Let them calm down and hopefully get curious.
- Aggressive - Charging with mouth open. Trying to attack you.
----- Block. Don't freak out and don't run off. They're trying to scare you away. Wait until they calm down before doing anything else like interacting or closing the tank.
- Evasive - Scared. Trying to hide. Can take off at high speeds. Can get defensive if you block them from hiding.
----- I usually let them hide if they're somewhere safe, and wait a moment before uncovering them and trying again. If they're in a constant defensive/evasive cycle, they'll get stressed, so if I can't get them to calm down much in the first few attempts I tend to back off and try again some other time.
- Relaxed - Staying still. Eyes may close. May "huff" if bothered. Can get defensive or evasive if startled.
----- This is good. Usually happens in the sun or when they've found somewhere warm/cozy. Let them be and enjoy having a relaxed animal. If you're approaching to interact, make some noise/movement nearby before touching them so they're awake, alert and know you're there.
- Stressed - Eyes wide. Rapid/heavy breathing. Lots of being defensive/evasive. May drool/pee/poop.
----- Time to back off and let them settle down. If they're in-tank, close up and leave them alone. If out-of-tank, put them in-tank or somewhere dark/cozy/safe. Go back to normal routine and try again once they've re-settled-in.
Other things to note:
- Accept the fact that you'll possibly be peed/pooped on, especially as you're getting to know each other. Carry a towel and when it happens, don't freak out. Clean up and continue on.
- Accept the fact that you may get bitten. Hopefully it's only a test or go-away bite, but they can in fact bite hard and hold on if they really want. It may hurt when you're bitten. If it happens, don't freak out. Wait it out and continue on.
- In both these cases, if you freak out or immediately put the animal away, it can actually cause a learned behavior. Being put away or having you leave is basically the animal version of 'winning'.
Also, knowing the weight of your animal and keeping track is probably the best way to know if they're healthy. If they lose a lot of weight, then you can worry about why. Otherwise, try not to worry. If they're eating/pooping/not losing weight, you're on the right track.
There's probably more, but this is what comes to mind as my advice. Good luck
But the first little while will require some patience and understanding.