One of the things you need to be aware of is that some of the services and things you sell are taxable and you didn't even know it. Here are a few from some recent assessments.
A trucking company got nicked for the sales tax they should have charged the contractor drivers that they sold the trucks to. The transaction was complicated, but they WERE selling the trucks and it didn't qualify as an occasional sale because they sold a LOT of trucks.
A time-share company that sold furnished apartments should have been charging taxes on the sale of the furniture. And the fact that they had already paid tax didn't count because they "used" them by having them in the units to show before the units were sold (they were "on display).
A boat club found out that its membership fees were taxable because they really were rental fees. And the rental of boats was taxable.