WHY?! WHY?!

Ok....I am very angry...I just defeated the boss in Kingdom Hearts 358/2 days, something like leechgrave. Then a cutscene and then my game froze! I didn't save so I am back to when I got my expert rank! What can I do to prevent this from happening again?!

By saving often, but the freezes might means some trouble on your DS.
Ask around the Technical Help section.

Do you have a patch on it?PATCH IT.It is in the Tech Help Board.And...
Try not to jiggle your card in the slot, it freezes games that way.
Use a piece of paper in between your card and DS to increase resistance.

Well I will save and when I get to Leechgrave (Which is like 40 days or something) I will beat him and see if it freezes.Oh and I am using No$GBA not a real DS.And yes I have the patch.

Then... the patch is not working.None of the patches are perfect, and it may just be an issue with NO$.I'm sorry for you though, that happened on my Platinum and HeartGold, I had to start 2 Gyms behind...

If you're using NO$gba, make sure the have the file firmware.bin in your NO$GBA folder and using the 2.6a version., this will resolve all of the game issues.

Please name your title more appropriately and post the topic in the right section in the future.