I've always liked to view a bar graph of my daily sales over the past 2 months. However, I found that my impression of "slow sales" can get out-of-sync with reality. The most helpful thing for me was plotting three lines over the bar graph: monthly average, 7 day simple moving average, and 2 month simple moving average. This makes it easier to spot short and long term trends. So now when I feel like it's a slow day I look at those trend lines and am reassured.