I honestly enjoy the WordTracker service. Google Analytics is great for analyzing what sort of traffic is coming to your site as well.
Realizing what sort of traffic is coming to your site based on the keyword is definitely helpful. Here’s a little of my secret sauce… you must make a differentiation between what is considered a “money keyword” and a “traffic keyword.” That’s common knowledge among most of the established affiliates, but if you’re just starting out, that wisdom will save you lots of time.
I like to convert my keywords at a certain percentage, depending on which niche site I’m using. So, you have to decide which keywords are most valuable to you depending on the traffic or “money” keywords.
Keep in mind that the type of offer and the profit margin has a great deal to do with how effective your keyword might/shoudl be.
Play with keywords, check out WordTracker and Google Trends and see what you can do. This is still a relatively new market in the general world of marketing. It’s a lot of fun to experience and try to predict, but you can really break down your keywords into what is profitable and what is not.
Here’s my final tip for figuring out what keywords to include in your efforts:
Look at the backlinks of your competitors!
Let me know if you need help!