Minimize the number of CSS files to reduce the number of HTTP requests. If several style sheets are used on all of the website’s pages, concatenate them into one single file.

Some CMS and frameworks offer ways to do such optimization automatically. The HTTP server can also be configured to compress and reduce the size of style sheets (see best practice #79).

Bitmap images often constitute most of the bytes downloaded, just ahead of the CSS and JavaScript libraries. Optimizing them thus has a considerable impact on the bandwidth consumed. The first step is to choose the correct format between bitmap (e.g. JPEG, PNG & GIF) and vector (SVG).
Bitmaps should really only be used for photos and interface elements that are not possible though icons or CSS styling.
The choice of bitmap format depends on the image’s characteristics: black and white or color, color palette, need for transparency, etc. For these characteristics, the ability to use lossy compression on the image is more suited to JPEG and WebP (Google); while the need for transparency and/or lossless compression is more suited to GIF or PNG.
Tools such as pngcrush, ImageMagick and jpegtran will help you reduce the size of images as much as possible.