Stephan Spencer writes a good simple tip on keywords.
‘To rank for the most generic (yet still relevant) keyword possible, your page content needs to be focused on one (or possibly a couple, but certainly no more than three) central keyword theme. Each page of your site should “sing” its own unique “song” (keyword theme) to the search engines.’
This is one of the key pieces of advice I am constantly giving bloggers. The way I look at is that each Blog should be focused on a niche – but that each post (page) within it should focus in on a sub-niche of the larger one.
To take Stephan’s ‘song’ analogy – see your blog as a compilation album (I’m thinking of some of my 80′s CDs) that is a collection of songs on a larger theme. Each song (post) has its own characteristics and focus that tie into a larger theme.
Whilst there is a place for the larger post that is a little more general in topic – its can often be more effective to break such posts up into smaller ones (a series if you like) and make each part focus upon one element (keyword).