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