Rather than exclude outliers, you can use a robust method of regression. In R, for example, the [`rlm()` function from the MASS package][1] can be used instead of the `lm()` function. The method of estimation can be tuned to be more or less robust to outliers.


  [1]: http://stat.ethz.ch/R-manual/R-patched/library/MASS/html/rlm.html