Gerald Prante @ Tax Foundation [340K PDF]:

For 18 consecutive years the Tax Foundation has published an estimate of the combined state-local tax burden shouldered by the residents of each of the 50 states. For each state, we calculate the total amount paid by the residents in taxes, and we divide those taxes by the total income in each state to compute a “tax burden” measure.
We make this calculation not only for the most recent year but also for earlier years because tax and income data are revised periodically by government agencies, and in our own methodology to take advantage of new datasets.
The goal is to focus not on the tax collectors but on the taxpayers. That is, we answer the question: What percentage of their income are the residents of this state paying in state and local taxes? We are not trying to answer the question: How much money have state and local governments collected?

Related: Milwaukee Journal-Sentinel Editorial on McCain & Obama’s deficit spending plans.