Use the same version of iReport and DHIS2’s version of Jasper reports. See the About page in DHIS2 for the Jasper version in use.
Use report tables with cross tab dimensions as your data source for your report designs. This will make it a lot easier to design reports where you need to put specific indicators, periods, or orgunits on columns.
Learn from others, there are many DHIS2 report designs for Jasper on launchpad, see http://bazaar.launchpad.net/~DHIS2-devs-core/DHIS2/trunk/files/head:/resources/