An unused agarose gel may be stored for later use. However, you need to check on it during short-term storage to ensure it is not dehydrating. Rather than dealing with the problems of a dehydrated gel--affected matrix and stronger salt concentration--it is simpler to just make a new gel. If you have already run the gel and used stains or dyes, you cannot store the gel. You must visualise it immediately as the dyes break down; once visualised, you discard the gel according to protocol.
Mix the agarose with a specified amount of buffer, heat to liquefy--swirling often--and then let cool to solidify.
Seal the flask thoroughly with parafilm, once the agarose has completely cooled.
Store at room temperature for two to three weeks or indefinitely at 4 degrees Celsius. This is a stock solution of premixed agarose gel--it will require reheating prior to use.
Pour the hot liquefied agarose into the electrophoresis chamber with the well combs in place and allow to cool and solidify.
Fill the chamber with buffer, covering the now-solid agarose gel, and remove the comb(s).
Seal the entire gel box and gel completely--with Saran Wrap or parafilm--and then refrigerate and store at 4 degrees Celsius. The gel may be stored indefinitely so long as it remains covered in buffer and is completely sealed. Should the buffer evaporate you will end up with greater salt concentration than desired, so the buffer should be changed prior to electrophoresis.
Remove the casting tray with the solidified agarose gel from the electrophoresis chamber after wetting the gel with buffer.
Wrap the wet solid gel completely--with or without the casting tray--in Saran Wrap.
Refrigerate for one to two weeks at 4 degrees Celsius.
Don't leave single wrapped gels in a refrigerator indefinitely--even wrapped, they will begin to dehydrate. Your concern with storing gels is dehydration. Discard if they begin to look dry at the edges or thinner than when you poured them. The only agarose that can be stored usefully after electrophoresis is separated nucleic acid excised as a band from the gel and stored at 4 degrees Celsius in a parafilm-sealed Eppendorf tube.
Be very carefully if you remove the gel from the casting tray; the gel is very fragile and will tear easily.