All discussions of Black Swans aside, whatever last month looked like is a good starting point for guessing what next month might look like. By all means measure each month and look for trends. Perhaps you and/or your organization is getting better and your velocity is increasing. Now we have more confirmation of that fact. Or perhaps in response to missed deadlines you’re attending still more mind-numbing meetings to discuss such gems as why things take too long and so your velocity is decreasing. Cold comfort perhaps but now you have data to back up that dark realization.
As an individual you might pick from several data sources to establish a velocity. The selection may depend on your job function (sustaining engineer, development engineer, etc) and/or on the systems you are using for bug tracking, source code management and the like.
You might track the number of bugs fixed by you, the number of source code checkin’s you do, the number of code inspections you participated in or some combination of these and others. One interesting strategy is to gather the data from as many interesting sources as possible and then graph the results for longest time period that you have data for. If we assume that your actual theoretical velocity has been relatively constant you can select a data source or sources that results in a flat graph. Of course if you have switched languages or processes or problem domain your velocity likely has not been constant so your mileage may vary.