The two more common ways are:
(a) NRaas Porter: This method preserves the relationships and history, but only of the sims who are packed together. The world they came from would then, within that timeline, no longer be playable.
http://nraas.wikispaces.com/Porter+FAQ (note the picture tutorial).
(b) NRaas Traveler to move them to their new homeworld. This method keeps the original world in play and connected within the same ongoing save just like the WA worlds. All history and relationships are preserved, the player can transition active play between the two (or more) worlds, move sims between them, go back for visits, etc.
http://nraas.wikispaces.com/Traveler+FAQ
