Search Engine Optimization (SEO) is a bit tricky business and some webmasters especially at the beginning of their web career are trying to take a shortcut. Here are several typical errors, which can be made in attempt to improve site rating fast, but usually leading to just opposite results.
Graphic Header It is standard and tasty to have a bitmap with a nice graphics on top of your page view. Logo, smiling faces or something else really eye-catching depending on the nature of the site is always nice to have there, your visitors would like it and expect it to be there. Unfortunately, SE robots has a different point. They assign highest value to all words at the top of the page, especially to those which are set under h1 tag. Best tactics: before giving this valuable space to wordless pics (words within graphic file are still graphics), to post h1 heading with crucial keywords above the graphic header.
Hidden Text In the process of placing keywords in their site many beginners eventually are getting the same "bright" idea: to place a large number of keywords invisible to visitors (using background color for the text, stuffing large number of keywords in no script areas of the code, and so on). It looks like a cool thing, but in reality it is very uncool! SE can catch it and will punish the site. In general, it is worth to remember that search engines do not like spamdexing (cheating with SEO) and their robots are specifically trained for recognition of such tricks.
Repeating h1 tag Text under h1 (heading 1) tag is getting higher priority in some search systems. For this reason some web masters may attempt to use h1 more than once. Bad idea, SE can punish for this too. It is better to use h1 just once, and use h2 and h3 repeatedly for the rest of all necessary headings.
Final Tip. The advice given here is for SEO only. So, please, dont forget that your site is supposed to serve people (hopefully), not web spiders. Besides, SEO is one of the major, but still not the only way of making your site visible. It is not a good idea to base your traffic on SEO only: a change in SE algorithms can cost you a substantial part of traffic. It is much better and safer to combine all possible means for attracting visitors to your site.
Good luck!