Use a cash drop box with a time delay lock and advertise this with a sign. Locate a cash register where it can be seen by passers-by to increase the likelihood of identifying the robber. Providing credit facilities reduces the amount of cash you need to hold. Requiring exact money in transactions means you do not have to keep open cash tills, especially at night. Robbers may tender large bills specifically to find out where you keep them.
Time controlled vaults reduce the opportunity for theft.
Advertise them with signs. An open and uncluttered environment which provides a clear, well-lit view of the sales area from outside is a deterrent to armed robbers. Minimise curtains, posters and advertising materials which obscure vision, as they provide cover for bandits. Bars on windows may be necessary. Get rid of louvres.