The easiest way is to go to both Amazon and Gtrends and see what's hot at the moment & is trending upwards, not much point in pushing a vertical that has traffic dropping off. You then do your keyword research as you would in adult and look for easy targets to dominate in the serps, or look for low competition/bid prices on your target phrase in both ppc (adwords,etc) and ppv traffic sellers.
Now you knock up your site and get the traffic flowing to it, either via organic search traffic or paid traffic as I said above. Now you have to decide are you going to collect consumer data and build your opt-in lists or are you going to sell something to the consumers directly from your site?
If you decide to build your own list (by using embeded opt-in box on your site or a pop/onload squeeze page) then you get multiple chances of trying to sell something to them. Good paid companies to use for this are Aweber and Getresponse.
If how ever you want to just try to sell something to your traffic from your site you have a few options here. Firstly you could try to sell a cpa offer, plenty of networks out there just check out Offervault. Second you might want to try dropshipping the product yourself but this will require you to do some work, I know effort heh. The upside to DS is you can use providers such as Alibaba and so on to do this for you. You just bill the customer and get the item sent out.
The upside to DS is you get a bigger cut of the item price, compared to the lame 6% or so if you would have sold the same thing via Amazon.
If you like low % payouts you can sell direct via the Amazon affiliate program.
There are also other cool things you can do with the traffic to your site. For example you could make a membership site that charges $7-$30 or whatever you like per month rebilling.
For example you might be targeting the male weight gain (yes that's right gain not loss) vertical. In your members site you might knock up a ton of information/tutorials/home videos on how you went about putting on weight. You could be a skinny fuck like me and blogged about how you stepped up the gym muscle gain and ate 10 meals of chicken a day to put on the kilos.
Inside your membership site (which you can protect with Amember and bill via Paypal) you could upsell a male weight gain cpa offer which most cpa networks carry, these often payout $40+ on a $2.97 shipping trial.
So by now you should see that there is tons and tons of different things you can do to make money online in mainstream. The best part is you don't need to sell shitty ebooks like all the skids/Indians do on DP and other lame forums.
You just need to get out of the "must sell porn, must sell porn, must sell porn" mindset.
-N