I know if I didn't thinks something was good for my daughter she wasn't getting it. But 1st make sure it's not a money thing, not everything can be affordable. Then follow the suggestions above about not nagging, being good, working hard on your grades, then show how the thing you want can be beneficial to both you and them. Finally show what you are willing to do to get it, extra chores, saving money, sharing it. If after all that the answer is still no, then accept that quietly and politely so that next time they have more of a reason to see things your way. Good luck.