Its is better to put your hands on your chest, when putting hands behind the neck people tend to pull themselves up with their hand which causes neck pains, and a less effective workout.
Try to keep looking up at the ceiling. It can be tempting to move your head forward because it makes you feel like you're lifting further, but you're not. It's a bad habit that increases your chance of getting injured. One good way to prevent this is to always try to keep looking up at the ceiling.
Keep a slow pace on the way down Don't just fall back, the lowering down works your ab muscles as well.
Don't go too high The crunches are made to isolate the rectus abdominus muscle (the six pack), going to high will engage your hip flexors making the exercise less effective. Make sure your lower back stays on the floor.
Crunches are pretty good strengthening your abs, but you might throw your back out in the process.
Crunches, or worse, sit ups, involve flexing your spine repeatedly, which get worse if you add load to it, like a weight plate.
Too much flexion is the exact mechanism that causes injuries like a herniated disc.
To stay on the safer side, try to hold your spine flat on the floor as you bend.