- Maintain an upright posture. Don't slouch.
- Attempt to sit on your hamstrings to make the low back stay tight.
- Perform the movement like a standard overhead press in terms of bar path. Press the bar over the spine.
- Keep the heels and backs of the knees glued to the floor. Try not to move them.
- Don't attempt to sit on your butt. It'll translate to a round back and an injurious pressing position.
- Don't lean back during tough reps. If you can't stay straight as an arrow, your set is done.
- Don't start with the barbell on the floor. Set up the pins in the squat cage a couple of inches below shoulder level.
Whereas a standing press allows the legs to stabilize the trunk, especially via a wider base, the Z press is performed sitting flat on the floor.
The knees are to be kept straight, and a slouch is strictly prohibited. If you try holding that position without any weight on the floor, you'll get an idea how uncomfortable and just plain hard it is to simply maintain the position.
In short, you'll need a whole lot of trunk strength, hip flexor mobility, hamstring flexibility, and lumbar and thoracic spine health to perform these bad boys well. And if you don't have every one of those things in check, the lift will suffer.
Remember, you have absolutely nothing to lean back against, and you have nothing to drive into since your feet aren't planted on the ground. That makes it a "no cheating" exercise.