1. Numbers are not neutral. I know this goes contrary to what you may have heard. You may have been told that "numbers never lie". But they do. They do all the time. This is because all numbers are subject to some level of interpretation and since it is human beings doing the interpretation they are subject to all the things that make us human, including confirmation biases, overly skeptical or optimistic thinking, etc.
2. Trends matter more then the moment does. Any particular sales day can be noise. It can be driven by a wide variety of factors that you barely control. So I always try to add the necessary element of time to my data analysis. A week is good. A month is better. If you have the luxury of several months that is even better.
3. Going into the data with a question that needs answering. When I'm look at a report I'm looking for answers to specific question. That question is almost never "what did we sell today" because that answer is obvious. I want to know if a particular strategy "moved the needle" or if it is even possible to tie a particular sales strategy to a specific outcome.
4. Remember that we are selling art. If PepsiCo called me tomorrow and asked me to be their Chief Marketing Officer I would be nothing but a data driven marker. That's because a Pepsi sold in Maine, Moscow or Minneapolis is fundamentally the same. When you have a similar product then numbers and trends can be very predictive of future outcomes. But I don't work at PepsiCo. I work in the arts. One play isn't the next. Purchases of one set of books doesn't necessary tell you what the next round will be. You can have all the data in the world and still be surprised at what happens when the curtain goes up. So use your data but don't fall in love with it. It isn't a crystal ball into the future.