Keep a bootle of pure grain alcohol in the snake room and NEVER go near a large constrictor alone. Always have someone nearby.
If bitten and constriction starts, the alcohol poured on it's head (it will get in the mouth that way) will make the snake release.
Make sure you do not smell like food.
Never lean into a large constrictors cage headfirst.
Use tongs to feed.
Buy a hook. Use the hook to tap the snake and make it aware of you whenever you need to work with it. When feeding it, NEVER have the hook with you.
You want the snake to relate the hook with coming out and no hook with food.
This guy wrapped me once and even though was small (only about 9 feet and thin), it was almost impossible to unwrap him with one arm free. Thankfully the wife was there and poured some grain on him. He let go right away.
With time, you can get to pick up on the snakes body language but since you are talking about rescues that may have been abused, they are far more unpredictable and you won't have the benefit of learning about the snakes habits as it grows. This one came to me after being underfed and poked with clothes hangers by it's previous idiot owner.