When you have a lists of numbers, you don’t need to use the SUM() function to get the sum of the numbers when you just need to know it at a glance. Just highlight the cells and the sum will appear in the status bar at the bottom of the window.
Summarizing data by month is very popular - but if you need to summarize data by month and your dataset bridges two years, your data will most likely be out-of-order or worse, summarized incorrectly as the group for “July” will contain 2006 and 2007 data. Here’s a function I use to print out the year+month so I can summarize across years:
=YEAR(A1)&”-”&IF(MONTH(A1)<10,”0″,”")&MONTH(A1)
That function will produce the year + month value in the format “CCYY-MM” and thus will always sort well in pivot tables or even just filtering.