The main idea is that if you use goals and then segment your marketing dollars into say 10 lots of $200. You can then run 10 different tests. You might run campain A on site 2 for a month then change to campain B on site 2. Different campains might work differently on different sites. The main thing is keep testing. For the first 3 months you might waste a bit of money but by the 3rd month you should be able to allocate at least half of your money to areas that work based on the tracking you've setup and then keep spending at least a little bit trying new sites. If you don't have at least some for testing new sites you will eventually start to saturate your market. Keep a log of sites that were ok because you might want to go back to them with a new campain targeting their users later on.