What about things like amazon referral links and that kind of thing?
For those you have to have a click-through lead to a purchase within the lifetime of the cookie (which is 24hours IIRC). For random ad placements it can be very hit and miss, for targeted ones it might be better, but still limited unless you go for the whole shop type thing or use them as supplementary to other ad sources. IMO.
For ads I think TLA is the best as, once you sell a link, you get guaranteed income every month. You can sell on single pages or whole site, and you can decide how many to sell. You also get to vet adverts/advertisers before accepting the sale. They're also very unobtrusive, plain text links.
You could look at affiliate schemes like commission junction if you wanted a wider scope.