When trying to discover theme keywords, there are certain tips that you should bear in mind so that you can successfully discover theme keywords that work for you.

Here are some of those tips:

1. Use stop words when they make sense. It is a bit difficult to narrow down what stop words should be used and when, but one thing you should remember is that the key word phrase may need stop words as part of the whole search term. This is because when you use a key word search term such as PRESIDENTS OF THE PHILIPPINES, it is more likely that people will actually input the entire search term than the abbreviated search term PRESIDENTS PHILIPPINES (in fact, hardly anyone would actually use PRESIDENTS PHILIPPINES since it does not make any sense.) It makes more sense to use PRESIDENTS OF THE PHILIPPINES in its entirety. That should give you a ballpark idea of how stop words should be used. It also helps you discover theme keywords that effectively use stop words.
2. List down any key words and key phrases which you believe would be used by other people aside from you. Unless you are a hermit on a desert island who has developed his own jargon that only you can understand, most people are in touch with what the rest of society would consider common key words and key phrases (either through deliberate exposure or through casual contact.)
3. You can use the keyword brainstorming software from Google called Adwords Keywords Tool, that can help you think up related terms to use on your web site. It also allows you to input keywords from your own list so that it can produce suggested terms that are related to your own terms. This helps people discover theme keywords when they have a hard time coming up with their own list.
4. Try to compute what percentage of Internet searchers will actually be interested in the search terms you come up with. At the start, this means using your own seat-of-the-pants ability to estimate the number or percentage. This helps you narrow down the search as you discover theme keywords that actually mean something to the most people. Remember, your aim here is to draw in the biggest volume of traffic that you can get to your site.
5. Another software which you can employ is the Keyword Discovery from Trellian. It can be used either as the free version, or via the paid subscription service. It can help you come up with core terms at the start and then discover theme keywords that effectively work in clusters.