a. Assuming it is not initiated by the player in their background, let it develop naturally based on in-game occurences.
b. Have the person or organization be something they encounter in game, preferably early on- like Rel wrote, the first adventure is always good.
c. Develop a reason (or reasons) for the character or organization to periodically show up. Perhaps they have opposed goals (seeking similar items, the same allies for an opposed war, religous antagonism, etc.). Maybe, the party killed one of their members and have placed a bounty or blood hunt on the party.
d. Don't be afraid to let the party suffer setbacks. If the party is seeking items they are after, the organization shows up and takes it from them as the PCs emerge battered, bruised, and low spells, hit points and other resources. Allowing the rival to escape is also a setback.
e. If the rivals can track the party, give them a reason that can be discovered. For example, in a previous campaign, the rogue, on the very first adventure, stole a ring from the corpse of a wizard they killed and kept it on his person. Only the rogue new about the ring's existence and for half of the campaign, the party wondered how the wizards' guild seemed to know how to find them. It wasn't until they caught another wizard with a similar ring that they learned that the the rings allowed the guild to track the posessor and view his or her surrounding area (the rings also allowed their wearer to teleport to a viewed location and back to the guild which explained how the wizards quickly appeared and disappeared).
f. decide if the the rivalry is friendly, professional or for blood and plan accordingly.
b. Have the person or organization be something they encounter in game, preferably early on- like Rel wrote, the first adventure is always good.
c. Develop a reason (or reasons) for the character or organization to periodically show up. Perhaps they have opposed goals (seeking similar items, the same allies for an opposed war, religous antagonism, etc.). Maybe, the party killed one of their members and have placed a bounty or blood hunt on the party.
d. Don't be afraid to let the party suffer setbacks. If the party is seeking items they are after, the organization shows up and takes it from them as the PCs emerge battered, bruised, and low spells, hit points and other resources. Allowing the rival to escape is also a setback.
e. If the rivals can track the party, give them a reason that can be discovered. For example, in a previous campaign, the rogue, on the very first adventure, stole a ring from the corpse of a wizard they killed and kept it on his person. Only the rogue new about the ring's existence and for half of the campaign, the party wondered how the wizards' guild seemed to know how to find them. It wasn't until they caught another wizard with a similar ring that they learned that the the rings allowed the guild to track the posessor and view his or her surrounding area (the rings also allowed their wearer to teleport to a viewed location and back to the guild which explained how the wizards quickly appeared and disappeared).
f. decide if the the rivalry is friendly, professional or for blood and plan accordingly.