Make sure you can see everything that goes on in your store. Keep counters low, no more than waist-high. Mount mirrors in corners so there are no blind spots. Make it hard to leave your store without paying. Place expensive items in the center of the store, away from exits.
Some stores put turnstiles at entrances to the only way to get out is through the checkout counter.
Arrange your displays so that missing items are easily noticed. Place small items in neat rows or patterns. If you must, fasten expensive merchandise and attach alarms. Announce and observe a policy to prosecute shoplifters. The threat of being caught, questioned by police, put on trial and maybe even put in jail, may be enough to turn most shoplifters away.