There are currently 3 known ways to move your world tendency up.
1. Kill a boss, this normally results in your tendency going up by 1
2. Kill Black Phantoms, some will make your tendency go down, but most of them increase it by 3
3. Kill a Primeval Demon, this increases your tendency by 3
The best way to get PWWT is to do something known as "bouncing", where you go to Pure Black then bounce to white.
To do this, at least one boss must be alive in that world.
Get Pure Black, then go and kill a Black Phantom. This puts you at -1. Then, kill a Primeval Demon, this will put you at +2. Finally, finish off with the boss. This will give you the last point you need to get to +3; otherwise known as PWWT.
Keep in mind that your World Tendency will average when you return to the Nexus, this includes Black Phantoms or Primeval Demons disappearing should you kill one and not the other. This means that to properly "bounce" your World Tendency, you must do all of this BEFORE returning to the Nexus. This means you WILL have to travel through most of the area.