For starters, equip yourself with a Bow and a couple hundred arrows, the Silver Bracelet (boosts soul gain by 20%), Ring of Avarice (another 20% boost) and the Spell Thirst (20% boost once more, but you'll need 20 Intelligence to cast it once).

Once you have all required items, go to world 4-2 and run straight to the platform above the first reaper. From here you can snipe him (3-4 arrows) and kill him before the Shadow Men have a chance to come near you. You'll gain 12K souls with all boosts active, once you do that, run back to the outside and suicide yourself off the cliff.

You'll start at 4-2 archstone and you'll have only to recover your bloodstain and repeat the process once more.

Alternatively, if you have enough intelligence to pull both Soul Thirst and SoulSucker (NG+) you can win a massive 19K per 1.2 minutes by doing this method.