When you use Google Adwords and can't be certain of what keyword matching option you must use, then you can enter all three variations you have in your keywords list. For instance:

Art Posters
"Art Posters"
[Art Posters]

After using them for some time you can check the clickthrough rates of the keywords and delete the ineffective keyword combinations.

Always use the negative keywords feature. For example if someone uses the search term "free art posters" then you should use the negative keyword -free in the Google appropriate field and in result your keyword "Art Poster" will not be displayed.

By doing that you avoid the freebie seekers who are not targeted in your offer and as result you improve your clickthrough rates and reduce the cost of your campaigns. You can also use similar with the "free" term such as:

-no charge
-no cost
-costless
-chargeless

and so on.

The keywords number factor. The age of creating an ad group and load 4000 keywords instantly is over. Although there is not a specific rule for how many keywords you should use, follow Google's recommendations on the amount of keywords. Google recommends: maximum 100 Ad groups per campaign and 750 keywords per Ad group.

Targeted but not over targeted. So many people loose money because of that reason. If you have yoga training center in Florida it's obvious that people from New York won't be interested in your services. So displaying a general term such as "Yoga" or even "Yoga training center" will display your ad on Google Adwords and prospects with no interest in your offer will click your Ad which means extra cost for your campaigns and not transformed into sales.

On the other hand if you use a specific term like:

"Yoga Training Center in Florida"

You have more targeted clickthroughs, targeted prospects and eventually more sales.

But there is a drawback on that tactic. If you are extremely targeted and use a keyword of over 6-7 words it's highly possible that no one will search for that term and have no clicks or even views for your Ad.

How to find keywords? You could use the Google keyword suggestion tool but because so many other Adwords users use it, the keywords from that tool are very expensive.

Other tools you can use are:

Overture suggestion Tool: http://inventory.overture.com/d/searchinventory/suggestion
123promotion Tool: http://www.123promotion.co.uk/ppc/index.php
Good Keywords: http://www.goodkeywords.com

These tools are free but if you want a more sophisticated and advanced option you can use word tracker paid service: http://www.wordtracker.com

Use always the plural form of your words. Don't use "Yoga Training Report" but also "Yoga Training Reports".

Exploit the misspelling errors. Research indicates the 12%-15% of the search phrases and words are misspelled. So don't use only the term "staff" but also "stuff".

Use hyphens. For example use both terms like: "Yoga Training Center" and "Yoga-Training-Center"

Use Acronyms: If you use the phrase "internet service provider" in your campaigns you can also use the acronym ISP in your Ads.

Use of domain names. This is powerful because it's the most targeted term you can find. If your website's domain name is www.yogatrainingcenter.com you must use it as keyword term.