Over the last couple of years, I've seen one module appear on almost every high profile Drupal site.

That module is Advanced CSS/JS Aggregation, or AdvAgg, for short.

AdvAgg really helps to improve the frontend performance of your site. This module really helps to improve the aggregation and optimization of your site's CSS and Javascript files.

Here are two videos that will help you install and configure AdvAgg:

The AdvAgg Module , Part 1

The AdvAgg Module , Part 2

These videos are part of our "Speeding Up Your Drupal Site" class.