Before you know what should be done to optimize your website and increase the usability factor, it is important to learn what you should NOT do.This list is not in any order of importance as all the points mentioned here should be taken seriously.
1. Using a "Splash Page".
During your endless voyage through the Internet you must have come across sites where you go to the main page and all you see is a beautiful picture with the two words of text saying "enter here." The picture is beautiful (maybe), but this is a disaster when it comes to search engine optimization. Search bots are looking out for texts, but generally there is no text on a "splash page." The main page of any website is the most important page, and it has the best chance of being optimized. If you clutter it up with images instead of spiderable text, you are messing up your chance of optimizing the site. Search engines don't appreciate splash pages, and many human users too are annoyed by having to click (again) to finally view the content. Avoid splash pages altogether if you can (unless your site offers Adult Entertainment for horny individuals who visit your site without ever having the intention of signing up for the paid services).
2. Putting Important Text, or even all Text as an Image.
I have seen moronic (or lazy) web designers produce a beautiful web page that has images and text combined to give the look and feel of a normal web page. However if you try to put your cursor over the text and select it, you can't do it because there is no text. The whole page has been rendered as a jpg or gif image. It looks good to the viewer, but the search engine robot can’t find any searchable text. Be careful if you are using some automated web design tool which turns text into images. Also while it is quite extreme to turn an entire page into an image, you should also avoid changing your important headlines into gif images. If you have a flash website, try not to make the content area in Flash. In other words, avoid a 100% Flash website at all costs (unless you have a signature URL like Gucci.com or michaeljackson.com). Its costly, takes time to develop and extremely difficult to optimize. However, if you are loaded with mucho moolah, then people are not going to question you. Instead they will simple term your stupid and weird behavior as “eccentric”.