A new way of optimizing your web site and pages with keywords and phrases
around the theme (niche) of the site (or page) has become very popular and
somewhat includes Latent Semantic Indexing concept. Read More on LSI HERE!

Optimising for primary and secondary keywords has been replaced
by optimising a whole page around a specific theme.

It is still kind of the same concept but different approach.

There is also a cool free advanced search feature from Google that
I use for finding theme keywords for most topics I want to write about.

First, Google Search Engine is using Latent Semantic Indexing
to rank pages by relevance.
(Latent Semantic Indexing (LSI) is a technology that looks at words
on a web page, and then analyses the words and their combinations
to decide what is that page’s “theme” or topic.)

Here’s an example.

Let’s say Googlebot analyses a page, and finds that the word “pet”
is top occurring word on that page. From that one word, it would
be impossible to decide what that page was actually about. It could be
about:

“pet supply”
“pet sitting”
“pet grooming”
“pet medications”
“pet insurance” ,etc.

Now search engine must look at combinations of words to decide what
a page is about.

If a page had the words:

“pet”
“apartment”
“rent”
“los angeles”

Obviously the page is about “renting apartment in Los Angeles with pet”.
Do you see how important the supporting words are to the theme of a page?

Now this means that if you are
writing a page about renting apartment in Los Angeles with pet,
you need to make sure the relevant theme words are on that page,
so the search engines can accurately identify the topic of your page.

An essential part of any content writing is the preparation. First you
need to identify quality, highly-relevant keywords and phrases that you
can create your page around. And this is NOT keyword stuffing,
if you write naturally, quality content, it will automatically contain
a lot of the relevant theme words, without you putting any effort.

here’s a simple writing exercise:

Try to identify 25 words you want to include, so the page is well themed.
Write the article naturally, and then check the keywords afterwards.
ou will notice that most of the 25 keywords were present, without even trying.
The rest can easily be inserted into the content,
by either exchanging them with another words, or creating another sentence to
include it. The end result is a page packed with words related
to your theme, yet easy to read for your visitors (humans).

It looks like a lot of work to write a single “unique” article or a page,
but the rewards are priceless.
It will bring high search engine rankings, and longevity in the search engines.
And you’ll be amazed at the number of search phrases your pages will get found for.

Now about Google’s free advanced search feature for helping find theme
keywords. Well, its an advanced search that uses the tilde (~) character.

If you place the tilde character before your search term, will find
results, not just for the search term, but also synonyms of the
search term.
Notice that all synonyms are displayed in bold and that is what we’re looking for.

These synonyms are words that Google obviously think are related to
your search phrase, and are a great start in finding theme keywords.

For example if you search Google for:

~food ~facts

(Notice that the tilde replaces all spaces in the search term, as
per the example on the Google help page).

Google will return the top 10 results, with these words in bold:

Food
Food Facts
Food Information
Food Trivia
Cooking Tips
Restaurants

Even though you wont find all words are 100% relevant,
they should be considered for inclusion on your page and are all important.

Once you have completed this step, go and analyze keyword results,
and use actual search terms for building up the list of theme words.
You can use our Free Keyword Suggestion Tool for that.

I suggest you read these articles to fill in the gaps:
Silo Structure or LSI (Latent Semantic Indexing)
Boost Your Rankings - Onpage Optimization Factor

Enjoy.