R Regression Diagnostics Part 1 By Vik Paruchuri.
From the post:
Linear regression can be a fast and powerful tool to model complex phenomena. However, it makes several assumptions about your data, and quickly breaks down when these assumptions, such as the assumption that a linear relationship exists between the predictors and the dependent variable, break down. In this post, I will introduce some diagnostics that you can perform to ensure that your regression does not violate these basic assumptions. To begin with, I highly suggest reading this article on the major assumptions that linear regression is predicated on.
Just like any other tool, the more you know about it, the better use you will make of it.