There are a couple things you need to think about when coming up with the keywords for your advertisement. Marketing costs money and you want to use the budget wisely.
Target your audience.
Think as they think. Identify what is important to them. What are the words they will be using and what is their goal?
Study the competition.
What are your competitors are doing to attract clients? What adds are they using? How is there comparing planned and what points it is highlighting?
Use keyword research tools.
The Keyword tool in Adwords is very helpful, it gives you the analytics on the keywords you are using and what words would generate better traffic. Tha will help you to pay only for what can be helpful to you in Advertizing.
Analyze the results.
As with anything, feedback is very important. It helps us learn from the results and see what can we improve on. What keywords did well and what words might not get much attention.