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.
I have to agree with the original answer. But the correct way to convince people is not to beg or plea. You should show that you deserve the desired item/result. But I don't suggest being overly nice, then it looks like you are a spoiled brat trying to get what you want. Just be nicer, and actually try to be better. By showing you deserve it, and then staying that way, your parents will trust you more, giving you more privileges.
I bet they would be pleased if you would ask them for some ways you can make money to buy it yourself.
save up enough money doing chores or get a little job.
Work really, really, hard and do your best in everything. Then sweeten them up and pop the question, "Mom, Dad? Can I have a green (or whatever color) gameboy?" That works for me.
say "daddy (then sit on, his lap and give him a kiss) then say can i get a dsi
you should promise to get better grades... and if you don't let them take it back
Tell them if not u will grow up to be a crackhead :)
ask themm
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
say please
I have the same question!
you ask them
make a deal with them and stick to it :)
tell them you want rats
come out of the closet
be a good kid
Tell them its a Spanish trip and your parents are allowed to go !