First factor is demand. Find a profitable market before you take any affiliate program. With a bigger market size, it will be easier to promote your affiliate products. Go to freekeywords.wordtracker.com and try to find a product that has a lot of searchers in the net.

Second is a commission incentive. Don't take any affiliate program that pay you 5% to 20% for digital products. You will earn more if you promote an affiliate program that gives 50% or more commission for each product sold. There are a lot of affiliate program who give more than 50% for commission.

Third is sales letter. All you need to do to promote an affiliate product is by referring people to the publisher's site. Then you just hope that the customer get convinced by the sales letter and then buy the product. So you need to choose an affiliate product that has a good sales letter. One of the best ways to tell if a sales letter is good or not is to read it by yourself and see how do you feel. If you don't feel persuaded to buy, then it is likely that many others will not, either. Your referral efforts can be waste if the sales letter doesn't convert well.

Fourth, the promotion material. The purpose to become an affiliate is to do less work and make more money. An affiliate program is already saved you some time and effort, but you would also want to do less work where ad-writing is concerned. Some affiliate program provides necessary promotion materials such as solo ads, ppc ads, sales letter, and even follow-up email.

Fifth, quality. This one is self explanatory. You may promote the product better if you really know that the quality of the products is good and you have tested it by yourself.

Another tip is cloaking your affiliate ID so nobody can steal your commission. You can go to TinyUrl.com to convert a long URL with your affiliate ID to a short one and you can hide that your link is an affiliate link.