My steps usually follow:
1. Play through the game normally. Don't worry about completing the Dex till after the Elite 4.
2. Go out and catch every single Pokemon that can be caught by normal capturing methods. This includes Legendaries if you choose so.
3. Scan your PC. Select any Pokemon that are close to their evolution stage and train them first.
4. Take this time to gather up any loose ends, such as Pokemon gotten in trades, Pokemon gotten by speaking with People, Pokemon from other versions, Pokemon that evolve through items, etc.
5. Find any remaining Pokemon. Chances are, these will be the toughest to level and evolve. Use the Elite 4 as the fastest means for EXP. and keep at it. Use EXP. Share, it really helps.
6. By now, you should have most of the Dex finished. All that remains will be Pokemon you neglected to look at or special event Pokemon.