Is there any proper way to fix a sample to adjust for known demographic overrepresentation? My spouse frequently works with (expensive, hard to obtain) data samples; for example route information for commuting bicyclists collected using a smartphone app.  More often than not, these samples suffer from some kind of known demographic over-representation that they'd like correct for various applications.
Mindful of Karl Roves' and friends "corrections" to "obvious" democrat oversampling for the Nov. 2012 election polls which led to rather embarrassingly incorrect predictions, is there any theoretically appropriate way of doing this?
I'm not even sure what to call what I'm looking for -- is this what in some places is called reject inference?
 A: This is the fundamental point of weighting a sample to population.  You weight each individual in your sample based on known demographic features of the population such that the weights of each demographic group in the sample add up to population totals.
See any book on sampling theory and practice - no, it's not reject inference.
I recommend Thomas Lumley's survey package in R and the accompanying book on complex surveys.  Even if you don't use R it is a great, clear introduction.
A: The answer depends on if the data is collected by probabilistic sampling or not.
If this is a case of probabilistic sampling, then there are many good books in the field of survey sampling that could help you. The best to start with would be Lohr (2010) Sampling: Design and Analysis, 2nd Edition or Särndal, Swensson, Wretman (1992) Model Assisted Survey Sampling.
If data is not collected by probabilistic sampling (the inclusion probabilities of units in sample are not known) this is another case. I have a feeling from your description this could be your case. Propensity score weighting could be the tool for you (it is used in web surveys). There are papers by D.B. Rubin and others - for example paper by Lee (2009).
A: if your spouse is doing his/her analysis on a sample of the collected data, then to adjust the results for oversampling you would have to know the proportions in the population of your target variable. 
It would help a lot if you told us what the Y variable is and how it's measured. 
For a binary target, if your sample is 50% good, 50% bad, and you knew that in your population the proportion is 5% bad and 95% good 
... (presuming all bads in the population are found in the sample), 
then a weight variable (for the sample) would look like this: 
WEIGHT (Y=1) = 1, 
WEIGHT (Y=0) = p(good)/p(bad), 
where p(good) and p(bad) are the proportions of Y in the population. 
For a detailed description read Naeem Siddiqi "Credit Risk Scorecards"
