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Do all the mini-games you can. You get 10-30 points for that, probably more the better you do. Visit as many people as possible and water their plants. Get as many Pokemon as you can out from the Dream World per session. That's a good 50 points each IIRC.
Serebii.net has a list of things you can do that increase the DW points. Those above seem the easiest.
keep resetting to your house and going through the dream isle forest before you pick a pokemon, play every mini game, harvest all the berries the next day, and plant as many rows of berries that is allowed. If you see that there is no more pokemon and berries appearing in the forest it's time to leave and pick your pokemon finally.
Always reset when you get to the tree. You can go through multiple times before pokemon stop appearing !