Consistent results can only be achieved with manual flash.
But for reasonably good fill-flash results in strong ambient light, the best way to go is high-speed-sync flash, most useful in situations where the background is bright and the subjects is in the shade and needs the extra boost of light.
Before any of this, though, be sure you understand the principles of fill flash:
http://www.planetneil.com/tangents/flash-photography-techniques/ Bit by Bit: A Digital Fill-Flash Technique for Improving Images | CreativePro.com