With clickbank it's best to start with article marketing to get things running so that you have some capital money to work with if you want to scale up to:
1. Hiring people to write for you
2. Venturing into ppc
3. Buying trusted websites to push your offers through, etc...
Because people are going to have to pull out their credit cards with clickbank, it is a good idea to do your homework on your market before you write even one word of content.
Because clickbank isn't like amazon and the average person isn't too familiar with clickbank, you will need to build TRUST with your prospect. And your presell has to convey this trust without being too pushy.
You can do things like building rapport; relating to them and understanding their situation to build this trust, etc.