The salt analogy is brilliant if you take the time to consider what salt does and how it works. First off - as you have said - it preserves. You try to say that there are more effective means now - but not during Jesus' time. Salt was the way to preserve.
Second, the passage asks of what use is salt if it loses its saltiness? In other words - of what use is it if it no longer acts like salt, if it no longer preserves the morals and teachings of God in the world? It is of no use! Therefore we are called to be potent - we need to be potent if we are to be of any use!
Lastly, in order for salt to be effective it needs to actually touch what its trying to effect. Many people try to evangelize by going out on a street corner and yelling at everyone and telling them to go to hell! This is ineffective because the target audience has no ties to you - they don't know you, and they sure are not going to want to after your performance! The most effective evangelism is up-close and personal, with the people whom you already have established relationships with. That's not to say its a guarantee by any means, but much more effective.
Its so simple and yet so brilliant at the same time! A better question that should be asked is how does this not represent what we should be? So what if his target audience wasn't Christians at the time of the sermon - does the message not equally apply?
