How Can I Prevent Sperm Loss During Masturbation Without Harming Myself?

Hi Im 20. i musturbat it every cuple of days, every day my body is being more and more week ,which is damaging me internally. hepl me i want to musturbat like i should not lose my sperm from my body. sperm comes true balls how can i stop them before reaching prostate ? can i use rubber band like something between bals and pines or can i put my thumb on my prostate stopping them out how can i do that