How Can I Complete Pokedex Very Easily in Emerald?

Keep at least 20 Pokeballs of any kind on you while traveling. Walk in long grass, caves and surf in water often. Capture every new Pokemon that you come across while going from city to city. Once you reach the Pokemon League, you should have a large chunk of your Pokedex complete.

Save your game in front of every Legendary or one-time Pokemon you encounter, such as Sudowoodo, Kyogre, Latios, Latias and Rayquaza. Restart your game if you don't capture them the first time. If you defeat these Pokemon, you will not be able to catch them. Without catching them, your National Pokedex will remain incomplete.

Trade with your friends and family often, especially if they have the sister versions to Pokemon Emerald. Pokemon Sapphire and Ruby each have Pokemon that are unobtainable in Pokemon Emerald. By trading, you will be able to get the Pokemon that you wouldn't have otherwise.

Complete the Hoenn Pokedex, so Professor Birch will give you the National Pokedex. With the National Pokedex, you will have access to new areas, new trainers and more importantly, new Pokemon. For example, Professor Birch will allow you to pick one of the three Johto starters.

Defeat the Elite Four and return to the Safari Zone. Two more areas are now open to you after your big accomplishment. In these two areas, you will now be able to capture second generation Pokemon, such as Mareep, Stantler and Hoothoot.