Since Google ranks your pages according to the number of links pointing at your page, it stands to reason that you should try to get as many links pointing at your pages as possible. This is so obvious that I'm only mentioning it for completeness sake.
If you have been placing images on your website without bothering to place ALT tags, now is a good time to add ALT tags. Google apparently indexes the text given in the ALT tags of images.
It has often been observed that Google considers keyword density a large factor in ranking pages in search engine results, more so than many other search engines.