I would go to powermat.com. There are some great diagrams of how these systems work and how they should be piped. The diagrams are for permanent installation, but the idea is basically the same. As rmeden said, the idea is high volume, low tempature rise. If you can put your hand on the panels when the system is running and they are hot, then you system is not running effectively.

Our panels have the input coming in at the bottom with all of the panels connected at the bottom, the last bottom tube is capped. All of the top panels are likewise connected, however the first panel is capped and the last panel feeds to return. The panels are also angle slightly with the highest point being at the return panel. This arrangement forces the water to go through all of the panels and prevents air from being trapped anywhere (bad, air doesn't conduct heat well).