Useful tips for PRP professionals and users alike.
Instead of slapping you should gently palpate a vein, here's how:
Gravity is your friend, blood like everything else is affected by gravity (one of the reasons injecting in your feet is higher risk). Try lying on a bed or sofa with the arm you want to inject with hanging down over the side. This should increase the amount of blood in that arm, and as a result the veins will appear bigger.
Another way of using gravity is using centrifugal force. The easiest way to do that is to spin your arm around like a windmill. The force on your arm will mean blood still enters but has problems getting back out.
With these techniques, and any other ways of raising a vein, it’s important to be confident they’ll work. So try them out when you don’t need to inject, it’s a great way to practice so that when you NEED to use them, you’ll know exactly what you’re doing.