There used to be a site called fueleconomytips.com that was "attacked" and taken offline. There were a multitude of articles that described the various methods of making gasoline engines more efficient. That site is no more.
MPG Research is a forum that has most of it, though it's spread out all over the place.
I know of no single modification that will make a significant impact on FE, though a few used together, if done correctly, can be greater than the sum of the parts.
I'm in the process of rebuilding my '79 Honda CB650 and am using most of the efficiency tricks. As soon as I can, I'll make a thread outlining the modifications and explaining how they work.