Reducing Data Transfers Amount:
you can reduce your transfer amount by;
1) building simpler websites.
2) more efficient websites.
3) optimizing graphics.
4) Remove unwanted tags, white space and comments.
5) Capitalize on CSS, call java script externally instead of embedding in every page.
6) Do not use too many keywords.

Alternatively, Avoid fancy flash presentations or streaming audio. Limit your META tags to those absolutely necessary. Besides many search engines will only review the first few and ignore the rest.
Another good idea is to cache your website but you might want to set an expiry date in the HTTP headers so the browser will refresh the content after a certain time. Use mod-gzip. It could save you as much as 40% of your bandwidth. Out of control robots can also suck down your bandwidth like a black hole. So use robots.txt to keep spiders in check.