I am designing rfid tag antennas(860-960 mhz) using hfss 12.1, can someone help me whats the best use of Mesh Operation function for increasing the accuracy of the simulation? thanks
Set it up correctly and it will be correct. For 99% of simulations, mesh operations are not necessary. Mesh operations tend to way over solve (university students tend to over use them), and then the user blames the software for excessive resource usage and solve times. Also, be aware of the measurement error for the antenna to be designed and do not force "accuracy" that exceeds your capability for measurement error.
As an aside, you will want to converge on re and im Z rather than S since your RFID antenna is likely being designed to match a strongly complex IC load. Look up expression cache in the HFSS help.