How to do multi-factor stratified sampling in R

Given a set of data with a number of variables plus a response, I'd 
like to obtain a randomized subset of the rows such that the
marginal proportions of each variable are maintained closely in the
subset to that of the dataset, and possibly maintaining as well the
two-factor interaction marginal proportions as well for some pairs.

This must be a common problem in data mining, but I don't seem to be
able to locate the proper library or function for doing this in R.

Thanks for any help.