Every time you beat the game you unlock a New Game +, where some things carry over. Each successive New Game + (you can keep beating it over and over) will be more difficult, and souls earned will increase.

Everything including weapons, experience and armor carries over into your New Game +, so prepare accordingly. Quest items, like keys, do not carry over.

A great place to farm is in 4-2, the Shrine of Storms. Equip the Ring of Avarice the Viscous Compound Long Bow +5 and Arrows. With the 4-2 Archstone and go down the hall and pass Graverobber Blige. Stand at the edge of the floor and fire downwards at the Reaper. Fall off the cliff, respawn, get your bloodstain and repeat the entire process.

When you die, hit the PS button quickly and quit out of the game before it auto-saves. You can re-load your game with all your Souls.

Be sure not to miss these rings:

Cling Ring: In the first level where you don't have your soul, after you get to the top of the castle, you'll fight a blue demon knight that's right in front of a fog gate. Beat him then go past him, not into the fog gate, and go down the stairs, when you reach the bottom there will be a lever to open the gate and you will find the Cling Ring -- it lets you keep more health when in soul form.

Thief Ring: After that first blue demon knight, go through the Fog gate, and you'll come to some stairs and you'll see over the ledge will be a gold knight (he is friendly and gives you rewards when you help him). Roll down the side of the stairs to the ledge and get to where he is. The Thief ring is behind him in the corner.