Pokémon Diamond and Pearl/Pastoria Great Marsh

Similar to the Safari Zone of previous games, the Pastoria Great Marsh contains many Pokémon that are unobtainable elsewhere at this point in the game.

Visit the Observatory
Before you enter the Great Marsh, visit the observatory upstairs. The people there can tell you how the Great Marsh works, and the binoculars by the windows will give you a preview of the Pokémon that you can catch in the Great Marsh. The binoculars will usually reveal the Pokémon of the day, which does not normally appear in the Marsh, but can be caught anytime that day.

Play the Safari Game
For 500 Poké Dollars, you are allowed to take 500 steps and throw up to 30 Safari Balls. You can't fight Pokémon here, so the only way to make them easier to capture is to throw bait or splatter them with mud. This may drive the Pokémon away, but if successful, the Pokémon becomes easier to capture.

Get HM 05 (Defog)
Speak to the Safari Game player near the gate. She has found HM 05, and would be happy to give it to you. You'll need Defog later, so be sure to challenge the Safari Game at least once!

The defog is needed in Celestic town and Mt. Coronet. While you can get through them without a defog (which might not be recommended), you may still need it at the Turnback Cave (after getting the National Pokédex) to obtain the Giratina.