Instructions
Fight every trainer. It can be tempting to avoid the gaze of certain challengers, especially when your team may be suffering the effects of a previous battle, but try to persevere, as the key to seeing every Pokemon in the Sinnoh region is to fight every trainer available. Keep plenty of potions, revives and antidotes to keep your team healthy and fighting fit, and if you have to avoid a trainer try to remember his location for the next time you return.
Return to previous areas that were off limits before. As you progress further you will acquire more and more HMs that will give you the ability to get to formerly unreachable locations. Climb rocks, cut down trees and surf the waves in order to find hidden areas where even more trainers and even more Pokemon lurk. One of the most important aspects of the Pokemon games is exploring as much of the region as possible, and in Platinum it is essential if you want to catch 'em all.
Beat the Elite Four. This will be your toughest set of battles yet and consists of a contest against four exceptional trainers followed by the Champion of the Pokemon League. Stock up on full heals, revives and hyper potions, and battle your way to victory. By this point your Pokedex should be nearing completion, but there are certain Pokemon, such as Vespiquen, Milotic and Spiritomb, that can only be seen here.
Fly to Sandgem town and talk to Professor Rowan. He will congratulate you for completing the Sinnoh Pokedex and reward your efforts with the National Dex, and a new route will open that leads to an entirely new area full of new Pokemon.