As Christians we are already saved, so why must we keep repenting of sin? We believe that Jesus died for all our sins, past and future, so why do we need to keep asking to be forgiven if Jesus already died to forgive that sin?
The general belief, that I think applies to most Christians, is that Jesus' death and resurrection opens the gates of heaven, saving us from sin in general, but that we must still choose to actually enter through the gate -- to repent of our individual sins and desire to sin no more.

To put it another way, Jesus' death makes repentance and forgiveness effective. Without His death and resurrection, repentance and forgiveness accomplish nothing.

Though each major tradition may have a slightly different basis for coming to this conclusion, it's easily validated by the oft-quoted John 3:16, particularly if taken with a little more of its context.


  16 For God so loved the world that he gave his only Son, so
  that everyone who believes in him might not perish but might have
  eternal life. 17 For God did not send his Son into the
  world to condemn the world, but that the world might be saved through
  him. 18 Whoever believes in him will not be condemned, but
  whoever does not believe has already been condemned, because he has
  not believed in the name of the only Son of God. (John 3:16-18)


Emphasis is mine. And really, the whole thing could be emphasized, because the general point is that Christ's salvation, though universal, is only effective for those seeking and trusting in it.