Tell them why you think you deserve it and what you're willing to do for them if they get it for you. For example, I've worked very hard in school, I've earned good grades, I work around the house. If you get this for me I will continue to be a good student and son/daughter. I will do my homework and chores before I pay the Nintendo and I will not use it for more than X hours a day during the week and x hours a day on the weekend.
If you're not a good student and don't help out around the house, maybe you should work on those things first and tell your parents you'd like your efforts, upon success, to be rewarded with the Nintendo ds. You could also offer to pay for part of it.
Basically, what do they want from you that you can offer in exchange for them getting you the Nintendo? Better grades? More help around the house? A better attitude? A clean room? Stopping a bad behavior? Spending more time with them/at home? Keeping your curfew? Being nicer to your siblings? There has to be something they want badly enough. Usually it's the thing you want to do the least. :) It may be worth it, though, if you want the Nintendo badly enough. Good luck.
if you want to convice your parents for you to get a 3ds tell them how awesome it is and then beg till you get it
Honestly, you can't convince them. Either they will let you or they won't.
if your parents wont let you get sponsored, well, id yell at em.
you say parents are going with
you ask them
I have the same question!
make a deal with them and stick to it :)
come out of the closet
tell them you want rats
be a good kid
say please
Tell them its a Spanish trip and your parents are allowed to go !