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My site doing well (first page in Google) for main search phrases (10K-20K searches a month for each phrase) but my niche is very spread so there are a lot of "second tier" phrases (around 1K searches a month fro each phrase) and my site is not doing so well there.
Even if I take a phrase (6-7 words appeared together, relevant to my niche) from one of my pages and try to search for it in a Google my site comes up ranked 40-70. Considering that I have exact phrase on my site I find it strange that other sites that do have those words but not together still come up first.
The page I took the phrase from has decent PR=5.
Example: if I search for "How do I install wide green widgets" (no quotes). Then my page with exact that phrase comes up far bellow pages that only have "install wide green widgets" words separately
So what can be done to improve the situation?
So what to do? Be attentive and intelligent about all the relevance signals - anchor text, title elements, etc. And beyond that, as with almost everything Google, backlinks and PageRank are a big deal in helping your url to outrank other urls that have the same words, even though in a different order and not in ideal proximity.
anchor text, title elements
And that's that.
Unless you're the only one seeing a generic connection between your 'main' and 'minor' phrases, and they're in fact apples and oranges to Google, you should be fine after correcting the above two.
Although I'd make sure you have the stuff in your body text as well. Perhaps in the META description.
As tedster mentioned, these are the basics, so you better get them right. I bet your nav is crippled, and your title doesn't really include anything that you want to rank for.