I have dealt with this a lot in the past
1. Dvide and conquer.. separate the staff if you can, and audit at the end of each shift
2. Show intent. I used to sling 17p in the till, expecting to find the till over by 17p. If they know you are spot checking, they will think twice
3. Have it out in a staff meeting, discuss the discrepancy, explain it is understood and being investigated
4. Look at your systems. SKU codes, stock checks, check random lines. make sure that the systems are 100% and water tight
5. use a mystery shopper.
6. start regular staff appraisals now, deal with all discipline issues immediatally, record everything, start a regime of BTW interviews and the like
7. install CCTV over the till, the doors and stock. IP CCTV tech is now cheap and can stream out of the building
8. invest in stock tags
9. implement a clear bag policy - everyone uses a clear see through bag, no handbags, no carrier bags, no shoulder bags
In other words, collectively, and managerially, get on their cases, and let them know you are on the case