I've been promoting CPA / affiliate offers for over16 years.
More specifically, I heavily promote PPL (pay per lead...lead generation) offers. This is because there is no credit card / purchase required. All a user has to do is fill out a form, so conversion rates are typically much higher than offers that require you to make a sale.
I favor offers that have a make, get or save money benefit to them. This is what has overall worked best for me and they tend to have the greatest mass appeal (will be of interest to a large general audience). So the potential exist to generate high volume and they are fairly easy to cross promote on the back-end.
Some of the PPL verticals that I have done extremely well with are: insurance, education, loans, debt, credit, assistance, mortgage, discount offers, homeowner offers, etc...
The bulk of the offers that I promote pay around $20-$40 per lead, but I also promote offers that pay more and less. However, you don't want to get caught up solely on what an offer pays because how well it converts is just as important. For example, if you have an offer that pays $9, but if it converts at 2X or more of a $20 offer, then it will perform about the same and possibly better. At the same time, if you have an offer that pays $90 and it converts poorly, it may not even be worth promoting.
Bottom line, it is far easier to get someone fill out a short form than to get them to pull out their credit card and make a purchase. So why struggle with trying to sell this or that, when you can provide free information that users want/need and get paid well doing it.
In addition to the above examples that don't fall into the make, get, save money angle. Promoting website signups (dating for example) can also be very profitable. I have done just as well with dating compared to the other verticals.
Free trials + S/H can work well. Weight loss is pretty evergreen. You really need to promote what is hot...For example, what is Dr. Oz talking about? You ideally want to promote something with a $50-$60+ (the higher the better) value and the user can get it for like $5-$9 S&H.
I promote a "very limited number" of products. I would stick with the above before trying to promote products, making straight up sales is harder. To make the same ROI or at least close, you need something that has mass appeal then it also has to be one of the following... 1. unique / not available locally. 2. something that the user can buy at a true decent discount. 3. solves a house is on fire type problem.
Anyway, that's what has worked for me.