micropipettes work with a system of springs, you should always store them with the highest loading volume (1000uL) in the 1mL micropippette (200uL) in the 200-20 uL pippette and so on.  this is because you are going to have the springs relaxed so they wont be any tension forces wearing them out. 

never tap the micropippette against the box of tips when you are taking one of them, it could loose the system of strings and plungers and decalibrate the pipette. 

at last you should always perform a calibration on your own, with very simple methods that are described in the 2nd link below, you measure ,load and discharge quantities of water that should have a determinated weight, do this many time to asure micropipette is working well. 

I love micropippettes, specially red ones, you can always tell where they are inside the cabine lol. 

http://www.pipettecalibration.net/pipette-calibration-files/Guide-To-Pipetting-2.pdf

http://g6pd.qap.tw/Pipette%20calibration.pdf

micropipettes work with a system of springs, you should always store them with the highest loading volume (1000uL) in the 1mL micropippette (200uL) in the 200-20 uL pippette and so on.  this is because you are going to have the springs relaxed so they wont be any tension forces wearing them out. 

never tap the micropippette against the box of tips when you are taking one of them, it could loose the system of strings and plungers and decalibrate the pipette. 

at last you should always perform a calibration on your own, with very simple methods that are described in the 2nd link below, you measure ,load and discharge quantities of water that should have a determinated weight, do this many time to asure micropipette is working well. 

I love micropippettes, specially red ones, you can always tell where they are inside the cabine lol. 

http://www.pipettecalibration.net/pipette-calibration-files/Guide-To-Pipetting-2.pdf

http://g6pd.qap.tw/Pipette%20calibration.pdf

All I can think of is to occasionally test that they are dispensing the correct volume (as 1 ml of H20 = 1 g you can dispense the liquid onto a micro-balance and see if it give the correct weight. i.e. 1 ul = 1 ug).

Thank both of you, very good answers!I'm going to calibrate them right away!

I use Gilson Pipetman Pipettes. Every 6 months or 1 year, our Institute would collect Pipettes needed to be calibrated and sent to Gilson company for service. Not only performing calibration for you, they also tell you which 'parts' inside the pipettes are wearing out and need to be replaced. This needs trained eyes or professionals. So, in my opinion, you should consider to have your pipettes send for professional maintenance every certain length of time.