Note: Before worrying about strengthening your bar, make sure you can do bars correctly and easily. For that, our "Bar Chord Mastery" course is highly recommended (there's nothing like it!).
Even long time players often suffer from an undeveloped, or improperly developed bar. Not having a strong bar, one that does not tie up the other fingers, which limits their mobility, limits all of your playing. Special exercises must be practiced over an extended period of time and practiced correctly, or you will cause big problems.
Here is a great exercise taken from a classical study, Villa-Lobos etude #2. I practiced this carefully for many years, and use it often to maintain strength. It will train the other fingers to be strong and independent while the 1st finger is barring. Try it yourself, if you do it right, and do it enough, you will see big changes in your left hand (directions below video).
Half way up
On highest note, notice the space maintained between 1 & 2
Coming down, notice 4 stays out