You will all be aware how important it is for pH meters to be accurate for BTEC, A-Level and beyond work for example determining an Acid Dissociation Constant. Firstly never store the probes in distilled or deionised water - use tap water. Secondly never touch the glass probe or it will be depolorised and always make sure that there is no bacterial groth on the probe by immersing in methanol for a few minutes. Thirdly allow the probes to sit in 3 M KCl for 30 minutes (especially for Silver-Silver electrodes. Next check the response by placing the probe in 0.1M HCl for 5 minutes then place in 0.1M NaOH for 5 minutes - the response should be fast. Finally calibrate the probes in accurately prepared pH4.00 and pH 7.00 buffers (place in pH 7.00 buffer first), check the speed of response and store in pH7.00 buffer until required (change the buffer every 5 days if the probes are frequently used, if not store dry). Good luck.