Tips to improve Linear Regression model
You can build more complex models to try to capture the remaining variance. Here are several options:


Add interaction terms to model how two or more
independent variables together impact the target variable
Add polynomial terms to model the nonlinear relationship between an independent variable and the target variable
Add spines to approximate piecewise linear models
Fit isotonic regression to remove any assumption of the target function form
Fit non-parametric models, such as MARS