Keywords are the main factor behind the concept of search engine. When people need to know something from internet, they look for the words in the search engine. While writing they use 3-6 relavent words, which defines the exact thing they need. Those particular words are treated as keywords.
1: The client's perception has to be understood.
Find out the requirements of the client and the words that are being generally used by the client inorder to search a particular subject. This will help you providing best result for them.
2: Do not use a single word, better to use multiple words.
Using a single word forms a boundary with limited options, but if that particular word can be added with some other words creating some meaning, can provide better result. The result interms can touch somany espects, by which we can reach to a number people.
3: Try to use simple keywords rather than marvelous keywords.
Generally, when a user search something in the search engine, he never use marvelous keywords. He always prefers for simple keywords related to his topic and the number of people using it is much more. So it's better to use simple keywords.
Always a web designer has to keep general people & their way of thinking, in his mind. So that he can develope acording to their requirement, which will help the site reaching maximum.