This is a short list. Please add to it.
Title metatag - Use your best key worsd here, not the name of your web site. Use it for 6 to 12 key words. It should be the first thing in your <head>
Meta description tag - Most engines use this tag. It should go under the titile tag.
Keyword metatag - Some engines use them some just check them. If they do not match the content it could be considered spam.
Key words and text - key words that are higher on a page are given more weight.
Headings - The seach engines also give more weight to <h> tags. <h1> having more weight then <h3>. So you should try and use these tags and use your best key words in them. It also helps if you use them in order <h1> then <h2>.
Bold - This is not as important as <h> tags but do help and taht is [b] tags.
Anchor Text - The links coming to your web site or to different pages on your site should use your key words in and around the link.
Alt tags - Use short sentences to describe what the picture is about.
Comment tags - Some engines use them.
Email addresses on every page - make sure it matches the domain name in the web site address.
Number of links pointing to your site plus how many links are one the page with the link pointing to your site.
Page Last Modified - A site that is updated often is looked at as a better site.