I've done quite a bit of coaching - I used to consult with a training program, and now of course AffPlaybook. In that time I've seen lots of affiliates who failed, or went on to make way more than I ever have. Here are my thoughts on what I've seen from the most successful affiliates.
1. They focus on 1 niche and 1 traffic source
A common problem with newbies is they spend $100 on PPV, then $50 on POF, then $75 on Bing. In that time they go from insurance, to dating, to credit reports.
I've never seen an affiliate who jumps around have much success. Successful affiliates start with 1 traffic source/1 niche, remove all other ideas/distractions, and make it work.
There is no 'best' traffic source. I've seen affiliates do great with PPV, PPC, Facebook, POF, Media buys, and SEO.
Likewise, there is no best niche. You want to pick something popular so it has enough volume.
2. Does it Scale
The top CPA affiliates run niches that have lots of volume, and on high volume traffic sources.
The first thing you should ask yourself when you pick a niche is 'how much volume does it have?'. In general, stick with popular niches like weight loss, dating, insurance, etc, etc. Niches like this have a ton of volume.
It's not that you can't make money promoting left handed can openers, but the potential market is limited. How many people are looking for left handed can openers vs. weight loss information or a dating site? Nothing against lefty's btw
Likewise, does the traffic source scale? How much could you potentially spend on it? If you can only spend a few hundred dollars per day, you're never going to be able to scale very far.
Again, you could have a profitable campaign with a low volume traffic source, but what is it really costing you? Is your time and money going into a campaign on a 2nd tier search engine where you make $50/day profit? $50/day is cool but if there's no potential to scale beyond that you're going to be stuck at that income level.
3. Keep Building Campaigns
I have a friend that was just starting out a few years back. He was bouncing around a little, with a little success here and there. One day on Skype he told me "I'm just going to start building 5 campaigns per day, every single day no matter what.". In about 2 months, he was making steady profit, and since then he's become a huge success.
The 'secret', if there is one, is to keep making campaigns. Don't fall in love with your campaign and think that every one will work. Be very methodical about testing - cut losers and scale winners.
4. There is a LOT of Money Out There
This ties back into point #2. Successful affiliates know what the potential is out there. I've never seen one ask 'is $100/day really possible?'. I'm not knocking the people who ask those questions, because I used to also. The point is that successful affiliates network, go to shows, talk to lots of people, and learn what the potential is out there.
This is a basic concept, but it's really just about knowing the potential out there. Who do you think will do better - an affiliate who wonders if $100/day is possible, or an affiliate who networks with affiliates making $5,000 per day?
5. This Isn't Easy
I'm often surprised at how easy people give up. They might throw $50 at a traffic source or campaign, and declare it a loser. It's easy to lose perspective when it feels like you're throwing up failed campaign after failed campaign.
Take a step back and look at how much you've really spent/earned. I bet it was less than it felt like. Affiliate marketing is a business like any other business - it requires an investment of time and energy. Even if you spent $5k without any return, you're still doing better than most business owners who spend much more than that getting their business up and running.
Successful affiliates don't look at their unprofitable campaigns as 'failures'. They look at them as investments in the business. Some also think of it like school - if you went to college you would spend a lot of money on your education right? I mean sure, you might get to act out your favorite scenes from Animal House, but you would be spending a lot of money to get educated.
Spending money on affiliate campaigns is similar - it's an investment in your education. Remember - the skills you develop in affiliate marketing can be used in many areas. I've known affiliates who took the skills learned from running affiliate campaigns and made successful businesses out of them. Consulting, creating a product/offer, creating a traffic source, creating a tool/software, etc. are all areas I've seen affiliates branch out into with success.
I hope this helped a little. I would be happy to answer any questions you guys have!