it's quite tough to get relevant ads sometimes. but here are all the things that have worked for me so far...
1 -- Don't overlook the obvious
Make sure your <title> tag contains your keyword, preferably at the start of the sentence
Make sure your meta keyword and description tags are properly filled out
Make sure your <h1> tag contains your keyword
Put your keyword at the start of the first paragraph, and sprinkle it throughout the rest of the body
Use a few <h2> and <h3> tags with variations on your keywords - but don't overdo it
Use <strong> <em> <b> and <i> tags to give weight to your keywords - but don't overdo it
2 -- Include keywords in the link text
Split your essays up into several pages, and then link to them using your keyword. eg: if you have a page about Man chester Utd (boo!) then link to a page of stats using 'manchester united stats'. Include 'manchester_united' in the filename as well
Use the title="" attribute within the links. eg: <a href="manchester_united_stats.html" title="Manchester United stats">Manchester United stats</a>
Make sure that you link to one or two pages on the web that describe exactly the same subject - from the same page. Make sure they all have a high google pagerank. Remember to use your keyword in the link text as well
3 -- Include some images
Include one or two images about your keyword. Make sure their filenames include your keyword
Remember to use the alt="" and title="" attributes on every image - including your keyword, of course!
4 -- Use Google's special tags
Help Google by using their special page tags. Include <!-- google_ad_section_start --> and <!-- google_ad_section_end --> around your content
Tell Google what to ignore by using <!-- google_ad_section_start(weight=ignore) --> and <!-- google_ad_section_end(weight=ignore) -->