1) Title Tag - The title tag is most powerful on-site SEO technique you have, so use it creatively! What you place in title tag should only be one thing, exact keyword you used for web page that you are trying to optimize. Every single web page should have it's own title tag.
2) ALT Tags - ALT tags were meant to be for text browsers because images didn't show in text browsers and ATL tags would tell visitor what it's about. You should put your main keyword(s) in ALT tags, but don't over do it because you could get dropped in results or even worse banned for life!
3) Link Popularity - Link popularity is most powerful SEO tool out of all them. Most search engines don't even consider web sites if there is not at least one or two links pointing to web site. Having another site(s) link to your web site is important when it comes to getting your site a good ranking. Your keywords should be in links you get and keep keywords short. When you receive requests for a link exchange, check site out before linking with them, check for spam (Repeat keywords, hidden text, etc.).
4) Keyword Density - This is also vital and should be used with research. You should use keyword(s) once in title tag, once in heading tag, once in bold text, and get density between 5% to 20% (Don't over do it!). Also use your keyword(s) both low and high on web page, keyword(s) should be in first sentence and in last one.