One thing that has always bugged me is how can two polls give different results to effectively identical questions and both claim to have "plus or minus 3%" (or whatever) margin of error, when their results have substantially larger differences than that?
Keep in mind that defining a representative sample is also fraught with differences. How you you determine if someone is a likely voter (are they registered? did they vote last time? other or some combination of the above?), for example, or ensure what you believe is a representative sample of different ethnicities?
Because a sample of 200 - 1200 is used and multiplied out to represent the population as a whole, tweaks in those definitions can lead to different results for the same questions.
I definitely second the www.fivethirtyeight.com tips, as well as www.electoral-vote.com for regular outside analysis of polls and discussion of methodological differences among the various polling places.
