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Always double count. Even when I think I'm right, I double count. I worked in cash handling positions for 10 years, sometimes handling tens of thousands.
When you're counting, try to do it in sets. So for example, if you're handling $1 bills, I count them "1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 20, 1, 2, 3, 4, 5, 6, 7, 8, 9, 30" and so on. This way, I'm not saying the "thirty" over and over again, which is where a lot of people trip themselves up. When you get to 50, put down the stack of 50 and start a new stack -- this way, when you're done, you know each of those stacks is $50.
There are apps out there that can help too, like this one. You can also find sheets like this. You can print it out, take it to work with you. Hope this helps, OP.