Well...... I finished the PokeDex myself so i got some helpful tips.
1) Trade. Trade, trade and trade. Go on the GTS, ask for Trades on this website, a place specifically made for Pokemon trades.
2) If you really want to complete the PokeDex, you would buy the previous games for the previous generations. I had Emerald, SoulSilver and HeartGold.
3) Get a friend to help you. I didn't have any games from the fourth Generation so i got my best friend to help me by completing Platinum's Regional PokeDex.
4) Have fun. Be sure you want to do this because it can get really boring after a while. Remember, the reward is close to nothing, only a virtual certificate and the pride and the right to brag.
Hope i helped you!