Keep your site design easy for people and search engine robots to find what they want. People want easy navigation, search engine robots want easy content.
One of the leading causes for sites not making it to the index is sloppy coding or "file garbage". Search engine robots work on a schedule; they enter a site with the clock ticking and if it takes them too long to find what they are looking for, they move on to the next site.
Next on the list is a lack of keyword content. Images are not content to the search engines UNLESS you add your keywords to the ALT tag for the images.
The size of your graphic(s) can slow the load time for your site. Make your graphics small in size by compressing them or set up a script to "pre-load" them. Keep your graphics in relationship to your text, GIF or JPG images used for headlines means that the robot has to look deeper in the file.
Using keywords in your META tags that is completely unrelated to the content on your site is a trick that a lot of webmasters use to get you to their site. This practice is a real turn off and best avoided.
Remember to keep things simple and clean. Many Websites are filled with too much clutter because the Site owner feels that it's best to give browsers everything he can find to throw at them. This is rarely the case.
Don't assume that the user knows what you're thinking... this means to make things easy to read, easy to navigate and have a visible link to your site map for them to find their way around if they get lost.