How to design flexible ERP app (report for government etc.)?

OK, sometimes I have to design and develop ERP software (accounting including), where some rather large number of reports required by government should be included - sometimes it may be simple to find solution for the present situation, but rules are changing frequently (at least once in 1.5-2 years) - and therefore it is temptation to find more flexible solution for each case (based on experience how rules have been changed in the past). How to find balance between current requirements and coding for future? Are there some general rules?

What are reporting (to government) capabilities of the enterprise level ERP&accoutning software (I mean SAP, Siebel, etc.) - how the leaders of developers of ERP and accounting software adapt their product for changes both - in government rules (reporting, tax calculation etc.) and in general accounting practice?

Any ides or suggestions where the interesting and practical resources can be found about this design matter?