Associated terms are often missed here.
Example. Ranking for "Cell Phone"
Google is going to expect to see terms like "3G, wireless, Apps, iOS, Android, Samsung..." in your content and a decent level. It is looking for these words to trigger that your content is in context.
I've found the best place to use the actual keywords is on embedded media. Such as the titles of embedded Youtube videos, the ALT tags of images and the OUTBOUND anchor text to authority sites.
Spamming the keyword in H1 and H2 tags is not advisable any more.
2.5% use in content MAXIMUM
Stay alert for compound word spamming. Falling foul of compound word spamming has lead many sites to be Panda slapped for words the webmaster did not even plan to rank for.
Example. Want to rank for "cheap blue widget". You count up the number of times "Cheap blue widget" is on page (remembering that each time it is used it is 3 words so using this phrase once per 100 words is 3% not 1%)
Then thinking that's ok, I've done well "cheap blue widget" is in at 2.3%, a great figure. The article talks bout how to take a normal metal widget and turn it blue, using blue metal primer or whatever. You write about blue metal primer, blue colourant and the "blue tinting process" or whatever bollocks. Before you know it you've over opped for the word "blue".
Not along with "cheap blue widget" but on it's own, in headers in other contexts. Before you know it your slapped for "blue"... a word you never even tried to rank for..
So remember. Count all the words in your phrase when working out the percentage.
Watch out for words in your target phrase appearing outside the target phrase too often.
Google doesn't give a damn what you are trying to rank for. It will slap for an over op of any word, not just the one you have in your mind to rank for.
They don't know what you are trying to rank for remember - and they always assume the worst of you and your motives.