Electrodes with liquid electrolytes (not gel types) may be stored either wet or dry.
A wet stored electrode allows an immediate use and a short response time, which is not true for dry stored ones. Unfortunately, the wet stored electrode is aging faster, because the process of aging (changing of the structure in the membrane) proceeds also in the case of non-use. Keeping electrodes wet should preferably be made in KCl solution (3M-4M). Most electrodes have a protective cap that can be filled with storage solution before placing.
To store pH electrode dry you must first remove internal solution, rinse the electrode in DI/RO water, and let it dry.
Note that you can't store dry combination electrodes and gel electrodes. In fact electrodes that can be stored dry are getting more and more rare.
If electrode is stored wet, don't forget to cover fill hole to prohibit evaporation of reference fill solution.
Gel type electrodes can be stored only wet, soaked in the KCl solution (3M-4M). Never store them in DI/RO water.
Check your electrode owners manual for details, as these may depend on the electrode make.