Start the keyword selection process by thinking big – don't restrict yourself by trying to formulate the most exact and narrow keyphrases; rather, come up with a large set of potentially relevant words and phrases that we can then whittle down to the optimal set. Look at the product or service; consider the language variants, the terms of art, and the related concepts. Make a list and add to it over time; this is one of those exercises that tends to benefit from having a bit of time to think about things and then process them. It's also a good idea to get other people involved to avoid subjective bias.
After you've assembled your preliminary list, it's best to take some time and do some research to make sure you haven't missed any candidate words or phrases. One of the easiest ways to search for keyphrase ideas is to look at what is being done by your competitors. To do this, simply identify a set of competing sites, visit their sites, and then view the source code to see what, if anything, is in their metadata. A useful tool for assisting with keyword selection is the Google AdWords Keyword Tool. It's free of charge and, despite the name, it's useful for a lot more than AdWords.