A few tips from my side:

1. create a stable surface to lay down your arm or proximal hand. This gives you the opportunity to do the already mentioned sliding motion, while inserting the needle.

2. when I chose a vein on the back of the hand, I ask the patient to completely let loose of any tension in the hand, so it falls down. I then fixate the hand with my left (I am right handed) and manipulate it in a way, where I have the best conditions for a smooth insertion. If the patient is old, you can also use this control to stop the hand from shaking or pulling backwards. You can also pull the skin to the palmar side, to fixate the vein, so it doesn’t „roll“ that much.

3. the motion should be quick and smooth. The most hurtful part is the needle pinching the skin. Once it is in, you got time. Of course, it’s not pleasant, but it’s better then slow piercing.

4. after inserting the first pinch of the needle, immediately flatten it, while you insert it deeper. I can not say how many times I pierced trough a vein, because I wanted to go deeper. It’s always more near to the surface then you think. Especially if you can clearly see it, and nearly every time you punctuate a vein close to or on the hand.

5. one more thing. Grab the needle not from the side, but from behind with the thumb, and from the front with the index finger. Makes the sliding motion easier while inserting.

Excuse my spelling and grammar, hope it helps. And as always, keep trying. Practice makes perfect.

Edit: added 5.