One thing you can do is look at the source code of similar companies. Near the top there are often meta tags with keywords in it. Here`s what you do:
Go to the site. In Internet Explorer go to View--> Source. (In Firefox it`s View--> View Source.) It will usually open up a Notepad page with a bunch of code on it. Usually near the top, within the HEAD codes you`ll see something like this:
<META NAME="keywords" CONTENT="incorporate, incorporation, incorporating, register business, registered agent, llc, llc formation, limited liability corporation, register company, incorporate online, articles of incorporation, company incorporation, nonprofit corporation">
Those happen to be words they are using as keywords for their site. (Not necessarily what they might be using for AdWords.) It might help you get some ideas though. Look at a bunch of sites` source code. I bet you`ll get lots of good keywords.