Make sure you have the latest Java update. Outdated java will cause freezes and crashes.
Close any other programs you are running for a smoother feel too.
Edit your game settings to lower the render distance, change graphics to "fast", turn off particles and turn off clouds, to see if that makes a difference too.
Also, if you are using an HD texture pack, turn it off to see if anything changes. Sometimes your computer may just be too old for newer HD technology.
Mods can also cause freezes if they conflict with each other. Always read the mod specifications before installing them. If a mod is the issue, you will have to delete your minecraft.jar folder and re-download it (by logging in again).
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A Nintendo DS game may keep freezing because the gamecard is dirty or worn out.
One of the Minecraft files that are required to start it may be corrupted. Uninstall and reinstall Minecraft and see if that fixes it.
There are many reasons why a Nintendo DS game may keep freezing, but the most common issue is dust in the game card.
They said that minecraft would never be a "finished product", and that adding more stuff periodically would keep users interested in the game.
/gamemode keepInventory true