[Agile] User story - what I have learned

Only one rule: clearness. 

It is not so important that a user story should be completed in few days as maximum, if this implies that it needs to become technical oriented.

An example
    We have a user story called 'multi currency' - actually our payment system manage only euro currency. This functionality is clear ; it is also big, so it is worth to think about some split, like 'multi currency only for protocol X'. A bad split is 'on database add a table named currencies'. Please don't do that.