Biggest reason people do not succeed is they give up to soon without putting in the effort required... or chase too many different opportunities at once and get no where with any.
And the bigger niches with the biggest potential take the longest.
Gone are the days of buying PPC traffic and making money on day one... it does not work like that any more in the world of Affiliate marketing. Adwords is a lost cause unless you are spending BIG money and Bing cannot drive enough volume.
When I want to compete in a even semi-competitive niche (selling my own product OR affiliate) this is what I do...
1. Invest in 5-10 aged domains..
2. Build out the money site with a well written content, reviews, etc... I will even purchase products to do video reviews for top recommendations - those can be used for video marketing - build a really good site that actually offers some value.
3. I will setup a network of feeder sites on the other domains spread out across a bunch of cheap hosting accounts.
4. Seed the feeder sites with 5 - 10 pages of good content (different lengths 500-1200 words + images) and then setup a drip of content to all sites - plugins to do this on WordPress.
5. Start ramping up link building to money site - low risk links to money site.
6. Same with feeder sites... but as they get indexed and links start to build up in month 2+ I will ramp it up and using more blackhat to build bulk to feeder sites - don't get greedy though. Need to be patient. Also make sure your linking profile is diverse... don't do too much of one thing. I do everything from nukes to xrumer to manual using outsourcing to buying.
This is the 5000ft view and there are lots of little details but unless you are going to jump into some obscure niche that has limited potential I don't expect to see good results for 6-12 months at minimum - if it happens sooner great but I don't worry about it. I know I am going to have to spend money to get this done and I won't see return for a while but if you do it right it will be pretty stable when it gets there.
So the moral of the story is you need to be patient and stay focused on one niche and own that niche... and there are no short cuts that will give you a long term, stable return.
Once you get there with one, use that cash flow to speed up the development of the next... hire writers, VA, link builders.
First one is the hardest... once you get the first one nailed, second one is easier.