Store it at 70 deg. If crystals form, stir,don't shake, to get them back in solution. A magnetic mixer is ideal for such mixing, as well as mixing all developers.
The solubility of poassium dichromate is ~100 g/l at 15C. So if the temperature drops below this then some of the chenical will crystallize out. Always check the solution before use and warm it with shaking to get the dichromate back in solution. Shaking will not harm the solution in any way.