Just like applying for a job, or a career, you need to know the pit falls of such and how to avoid them. Below I'll explain some of the more common ones (yes I've made these errors too) and what you should do to make your product choice a more profitable on.
FIRST ERROR:PICKING THE WRONG PROGRAM Now keep in mind, there are not BAD programs (well there are, but not many). Many people chose a program that is popular. Rage of the net! They may not even know what the product does, or, worse yet, they just scroll down to the affiliate link, grab their banners and way they go. Bad Move! First off, if your in any type of sales (thats pretty much what affiliates are), how can you sell a product you know nothing about. SOLUTION: Pick a product you know something about. This is good a for many reason, but most importantly - you will be able to sell its high points and know all the ins an outs of the product. Why people buy it etc..
SECOND ERROR: THEY APPLY FOR MANY AFFILIATE PROGRAMS: The one thing., among others that makes affiliate programs appealing is that they are easy to join. Because of this, people are tempted to sign up for many of them and try to sell them all. This will not work. Case and point: I am a card magician as a hobby, been so for over 30yrs. My mistake was trying to learn as many card tricks as possible. I would get the gist of one then learn another. Problem? I did not master ANY of them and as a result my performances were, lets say, less than entertaining. So over the years I worked hard on one trick at a time. Learning the ins and outs of it til I had it mastered. Take the time to learn about your product the same way. Make a profit from one, then sign up for another. Master each one. Trust me, affiliate marketing is not going away anytime soon
THIRD ERROR: NO EXPERIENCE WITH THE PROGRAM One of the biggest mistakes, not only in affiliate marketing, but in the sales world in general, is salesmen don't know about or use the product they are selling. To me that is a cardinal sin!! I'm not saying that if you sell sports cars you should be driving one, but surely if you are promoting a program that you should have least tried it, or are currently using it.
This is needed simply because you are more convincing to your customer if you are talking about something you KNOW about. In gaming, I'm heavy into Guild Wars. I know about leveling, what characters work, etc. I used to sell leveling guides for guild wars and world of warcraft, however, I do not anymore since you can find boot leg copies around the net. Yet many people that have NEVER played a MMO, are out there selling leveling guides. Only because WoW is insanely profitable.
So with all that, the bottom line is this: pick stuff you know about very well. That you have tried them and are very comfortable with its workings. And even though you know much about a few of them - make sure you are successful with one, before adding another to your portfolio.