If you are using Google AdWords you may be using their conversion tracking. Conversion tracking works well when you control the landing and conversion pages to plant the tracking code. Here is a guide to google conversion tracking.
However, the above may not apply to you. You may be promoting others such as affiliate programs, or do not have the ability to use the tracking code or prefer to use your own tracking code so you may determine a customers life time value. What do you do?
AdWords Destination Links
First, realize that in Google AdWords there is a difference between the Display URL and the Destination URL. The Display URL is the URL that is shown to the user while the Destination URL is the URL that the user will be sent to when they click. They must have the same top level domain, but otherwise they can be unique. As an example you may see the link http://longest.com but the destination may be http://longest.com/page1.html?var=goog
Unique Links for Individual Ads
Using a screen shot from Google Adwords Editor you can see the display URL and destination URL. This allows you to assign a unique link for each advertisement. You could use a variable for tracking purposes such as: http://longest.com/page1.html?var=ad1.
Unique Links for Keywords
Using another screen shot from Google Adwords Editor you can see the destination URL for an individual keyword. This allows you to assign a unique link for each keyword. You could use a variable for tracking purposes such as: http://longest.com/page1.html?var=keyword1. This would allow you to determine which of the many keywords you may be using are effective.