Over the last year (14 months) I've had my best fuel economy ever using a set of Bosch Platium IR Fusion plugs(Bosch p/n 4503), which are basically the same as the Platinum +4 except the center electrode is Iridium.
I was consistantly getting 32MPG+ tanks all year, with numerous 34MPG tanks, a couple of 35MPG tanks and even a 38MPG tank going to Ottawa---36MPG on the way back. It's crazy considering that the highway fuel economy rating for the car is 34MPG (and I'm not driving 100kph...).
Previously, I used the factory-spec Bosch Super copper plugs with dual-ground electrodes (F7LDCR). They worked well but fuel economy was much better with the IR Fusions. But the IR Fusions are more prone to cold-fouling if you start then stop the engine quickly (like moving cars around a driveway) but clean quickly.
But if you're getting foulled plugs (and they're not worn) is it your driving style? (more city than highway?) Do you idle excessively or let your car warm up? If not, than maybe your injectors aren't spraying properly.
For the record, I only used Shell V-Power and I literally started the car, drove up the street, and then onto the highway, then exited off the highway to work so I was operating it in ideal conditions to get these results. And the V-Power really does work, as every time I pulled the intake manifold off, the intake valves were shiny clean with no deposits.