Saving your game regularly is paramount to avoid losing progress. Pokémon Diamond and Pearl do not feature an auto-save function, so it's entirely up to you to ensure your adventure is recorded. To save your game:
- Press the X Button to open the main menu.
- Select 'Save' from the options.
- Confirm your choice when prompted.
It's a good practice to save before entering new areas, challenging Gym Leaders, or attempting to catch rare Pokémon. You can only have one save file per game cartridge, so be mindful of your progress. If you ever need to restart, you'll have to delete your existing save data, which is usually done by holding Up + Select + B on the title screen, though this is not recommended unless you are absolutely sure you want to erase your progress.