There is a certain set of things you should consider during Iteration Planning when breaking stories down into tasks. Here's a short check-list I created a while back to post in the XP-lab as a reminder. Other teams have done the same but with different items to consider. In our retrospectives we notice things we should have thought of and those items get added to this list.
Consider:
- Acceptance tests
- Risks
- DB impacts
- Design sessions needed
- ...
- Error conditions
- Boundary conditions
- Time-zero
- Front-end/back-end impact
- Permissions
- Audit logs
- ...