Your keyword lists are the root of a good PPC campaign. You should properly research and source quality keywords. Create a good long list. Go deep and go wide, by this I mean investigate every avenue and every avenue off every avenue etc.

Here's a quick example of how to expand your lists by investigating related avenues. In this example I'll use mortgage as the base keyword.

Mortgage is a very competitive keyword on the Net simply because the commissions are so high. So how do you break into the market without paying over the odds?

Create your first list and think of different ways your prospect could interpret mortgage e.g. buy house, bad credit mortgage, best loan, house purchase finance, no status loan, home finance, etc. This really is only slightly removed from the base keyword, you should dig deeper still.

Next try expanding into types of mortgage flexible, interest only etc. take these and mix them up with your first list, as you do this youll find avenues of keyword groups present themselves from single keywords e.g. interest only house loan, flexible home finance etc.

Avoid the general keywords and dig a little deeper to expand on related terms or, heres a good one, rephrase those terms e.g. turn cheap mortgage around and use mortgage cheap too. Put yourself in your prospects shoes. What would they search for? How would they type it into Google? People don't always type a search phrase in a logical order.

You could create an Ad group focused entirely on mortgage company names take that further still and add a .com to the name or a co.uk or your own country extension e.g. www.somemortgagecompany.com.au It doesnt matter if this domain doesnt exist, people type things like this into a search engine so its possible you can pick up low cost traffic with these keywords.

For some quick avenues to explore check out these keyword goldmines

Plurals, domain names, serial numbers, product numbers, spelling errors, no spaces between the phrase words e.g. buywidget. Add .com or .net or .info etc, company names, company domain names, domain names and variations e.g. mydomain.com www.mydomain etc. Add superlatives e.g. best, cheap, cheapest, best buy. Competition site names, foreign words, US or UK spelling, product names, typos, hyphens instead of spaces.

Be sure to separate out all these keywords in to different lists. You should be setting up different Ad groups for each. It's important to not just drop them all into one campaign together. Instead place all the typos in one Ad group all the plurals in another and so on. This will make for easietr targeting of your ad writing depending on your Ad groups keywords. Plus tracking results will be simpler.

Another great source for keywords are your site Log files they'll tell you exactly what keyphrase has been used to find your site by a visitor.