A common anti-pattern for Scrum and XP teams is to break stories down into tasks, track those tasks, and at the end of the iteration notice that all tasks are done, but the user stories aren't (because they are more than just the sum of their tasks).
I highly recommend not tracking tasks at all. Brainstorm them for estimation, if you like, but always estimate and track whole stories. If a story is to big, break it down into smaller stories - that sometimes takes some creativity, but it's almost always possible.
You can use sub-issues in Jira to aggregate stories into bigger stories, although this isn't very well supported by greenhopper, as far as I remember. If your team is colocated, I would very highly recommend index cards on a white board, anyway - even additionally to Jira, if you have to (that's how we currently work).
