Place your sign at bonfires as you progress through the game.Note you must be reasonably close in level, so place your sign in an area you have previously (or not yet) cleared.You will receive half the souls from the boss and anything killed along the way. Using the White Sign Soapstone and assisting other players in killing bosses is a fun and productive way to get souls. The Homeward miracle is great for cutting times off runs, but if you don't have it or can't use it you can use Homeward Bones at a cost of 500 souls each from Undead Merchant Female, weigh the time saved and the cost of the bone against the number of souls you are generating each run to determine if it is worth using them or running back.Many enemies will drop items, they can be fed to Kingseeker Frampt afterwards for additional souls.
This page contains different methods of efficiently obtaining standard souls to purchase items, upgrade equipment and level your character.