Well, having a directly East-West mooring allows me to have a panel facing due south, which probably makes the most difference.
The panels themselves are black-painted pipes, with black metal fins welded on (I would have used evacuated tubes, but couldn't afford it at the time). They're in heavily insulated boxes, with toughened double glazed glass & I put the whole lot as close to the calorifier as I could.
Apart from that, water is circulated by a central heating pump. There's a differential thermostat (from ebay), that turns on the pump when the water in the panel is about 8 deg. hotter than the water in the calorifier & closes a shut-off valve when water isn't being pumped, to prevent thermal circulation.
I think this is fairly standard ... Unfortunately, I suspect that success depends more on location than anything else.
B.B.