Hey Stefan,
Whenever I've had particularly skittish snakes, I'll put them in the bathtub with about 2" (5cm) of tepid (~90F or 32C) water and let them swim around for a minute or two. Before long they start to get a little tired and swim around the edge, trying to find a way out, but they can't because it's too slippery. Then I put my hand in front of them and let them swim into it. I let them rest. Then I put them back in the water and repeat the process another time or two. If they're a bit tired afterward, I can usually hold them while they relax in my hand. This way, they can get used to my scent while they're calm and resting.
If they don't relax after this first trip to the bathtub, I put them back in their cage and repeat the process a few days later. It doesn't take long before skittish snakes settle down. Not only does it let them associate you with safety, but it cleans them at the same time. Just make sure the water in the tub isn't too warm or too cold. Warm water (>32C) is notorious for simulating them to defecate, though, so have a trash can ready in case their tail begins to curve at the vent
. It isn't a defensive reaction in this case.
Rick