Before setting off to sell ClickBank products, make sure you verify the product isn't utter crap. Also make sure the sales page is strong enough to make you want to buy the product. These are two mistakes I made when I first got started.
I spent a ton of time with keyword research, making videos, doing social bookmarking, etc... All this, to sell a shitty item. Crap items do get refunded back.
Don't try to sell a great product that has a lousy sales page, unless you're willing to create a better sales page!
You can make the item sound as great as you'd like, but if the customer is completely put off, by the quality of the sales page, then you just lost a sale.