I agree that longer handling sessions could be beneficial.
While handling your snake watch closely so you can learn his body language, mannerisms, etc...
Watch the way his tongue flicks, watch his lungs expand and contract.
Also handle with confidence and respect. Snakes are good at picking up stress, nervousness or anxiety on our end and will often be stressed if they sense we are.
Also handle with respect. Understand that at the heart of things, snakes are simple, primitive animals. They don't know much more than "eat and don't get eaten". They only have their mouths as self defense, and hissing is their way of saying " I'm not comfortable in this situation". From there, we need to learn their body language.
Take your time, be gentle, try to always end handling sessions on a positive note and gently let the snake slither off your hands when you return him to his enclosure.
One last thing... I've never done this, because I've never had a snake that I needed to with, but I've heard and read it can be very helpful and usually successful.
You can try putting a t-shirt or something of yours in the enclosure so that it familiarizes with your scent.
Good luck. Feel free to ask any questions you have.