We found the leakage current made great effect in high resistance measurement. We used DMM4071 and PXI-2530 to made the test system.
How can avoid the leakage current?
Try to get hold off the 'bible' for low level measurements: the Keithley Low Level Measurements Handbook is available for free, just apply for it at http://www.keithley.com/news/corp100604. In this booklet you'll find all sorts of tips and tricks for advanced low level measurements.
I just found out there is a pdf version available: http://www.ntb.ch/Pubs/sensordemo/wtm/m_op/LLHB_6_Keithley.pdf